feat: a cell keeps its own margins where they differ from the table's

w:tcMar and an ODF cell style's own fo:padding were both dropped: the
table's value (in ODF the first cell's) stood for every cell.

cellPadding is now an attr on tableCell/tableHeader too, measured against
the table's rather than the editor default, so only a real disagreement
carries one. --cell-pad inherits, so the cell's own declaration wins for
its own td and the CSS is unchanged.

Checked against LibreOffice: a 1.2cm cell in a 0.3cm table puts its text
at 332.2pt where the grid column starts at 297.6.

docs/architecture/tables.md

@@ -12,7 +12,7 @@ Covers the `table*.ts` extensions in `src/lib/editor/extensions/` and the table-
 - **`tableSplit.ts`** (`TableSplit`) — Word/LibreOffice "Split Cells…": adds `splitCellInto(cols, rows)` (prosemirror-tables only ships `splitCell` = un-merge). Reads the grid via `TableMap`, expands it (duplicating the cell region's right-edge column / bottom-edge row so every other cell crossing a new grid line grows its span — the bridging Word does), partitions the region into `cols`×`rows` sub-cells (first keeps the original content), recomputes `colwidth` weights, and rebuilds the table node in one step. **Merge** uses extension-table's built-in `mergeCells` (drag/shift-click `CellSelection` via the always-on `tableEditing` plugin). Both are surfaced by `TableToolbar.svelte` (Merge button + Split button → `TableSplitDialog.svelte`).
 - **`tableCellBackground.ts`** (`TableCellBackground`) — adds a `backgroundColor` attr to `tableCell`/`tableHeader` (rendered as the cell's CSS background), set via extension-table's built-in `setCellAttribute` (so it applies to a whole `CellSelection` or the cursor's cell). `TableToolbar.svelte` exposes it via a reused `ColorPicker` (cell shading); round-trips to ODF `fo:background-color` (export/import below). Survives split/merge (attrs are carried through).
 - **`tableCellBorders.ts`** (`TableCellBorders`) — Word/LibreOffice border control: per-side attrs `borderTop/Right/Bottom/Left` on `tableCell`/`tableHeader` (`null` = the table default 0.5pt black, `'none'`, or canonical `'<W>pt solid #RRGGBB'`; rendered as per-side inline CSS — pt→px, min 1px — plus `data-border-*` for HTML parse). `setTableBorders(preset, spec)` applies a preset (`all`/`outer`/`inner`/`innerH`/`innerV`/`top`/`bottom`/`left`/`right`; spec `null` = no border, the default 0.5pt-black spec normalizes to `null`) to the selected cell region (`selectedRect`), writing **both sides of every affected boundary** — including the facing side of neighbour cells *outside* the region — so collapsed borders never disagree between adjacent cells. Surfaced by `TableBorderPicker.svelte` (dropdown in `TableToolbar.svelte`: preset grid + line width + line color; presets apply with the picker's current pen settings). **Word-like button states**: `activeBorderPresets(state, pen)` marks a preset active when every boundary it targets renders exactly the pen border — width *and* color, where a boundary's effective border is the collapse winner of its two facing cell sides and attr `null` counts as the 0.5pt-black default — so a fresh table lights the presets up under the default pen and a thicker pen turns them off; `'none'` is active when the region is borderless. Clicking an **active** preset toggles those borders off; an inactive one applies the pen. The panel stays open on preset clicks so the states visibly update. Round-trips to ODF `fo:border-*` (export → odf-kit `CellOptions.border*`, `null` falls back to the table's default border; import via `StyleResolver.cellBorders` + `borderAttrFromOdf` — an undeclared side ⇒ `'none'`, the 0.5pt-black default ⇒ `null` with unit tolerance, non-solid styles coerced to solid) and DOCX `w:tcBorders` (import also resolves table-level `w:tblBorders` by cell position; sides nothing declares stay `null` = default).
-- **`tableCellPadding.ts`** (`TableCellPadding`) — the table's **cell margins**, a global `cellPadding` attr on `table`: `[top, right, bottom, left]` in cm, `null` = the default `[0, 0.19, 0, 0.19]`, which is Word's own and what `editor.css` falls back to — so an ordinary table carries no attr. A side within `SAME_CM` (0.005cm) of the default *is* the default, or Word's 108 twips (0.1905cm) and LibreOffice's 0.191cm would tag every table. Rendered as one `--cell-pad` custom property (the attr's `renderHTML` for the static paths, `TableView` for the node view, since it bypasses `renderHTML`) that `td/th { padding: var(--cell-pad, 0 0.19cm) }` reads. **DOCX**: `w:tblPr/w:tblCellMar` direct, else the table style's (`DocxStyles.tableCellMar` walks `w:basedOn`); export writes `Table.margins`. **ODF** has no table-level cell margin — it sits on each cell's style, so import takes the first cell's `fo:padding*` for the table and export rides the value on every cell as a CSS-style TRBL shorthand that `expandCellPadding` splits per side (odf-kit only writes the `fo:padding` shorthand, and ODF allows it only one length). Getting this wrong is not cosmetic: a 0.71cm bibliography marker column lost 0.38cm to the old constant and wrapped `"[2] "` onto three lines.
+- **`tableCellPadding.ts`** (`TableCellPadding`) — **cell margins**, a global `cellPadding` attr `[top, right, bottom, left]` in cm on `table` *and* on `tableCell`/`tableHeader`. `null` = inherit: a table's from the default `[0, 0.19, 0, 0.19]` (Word's own, and `editor.css`'s fallback), a cell's from its table — so `cellPaddingAttr(sides, base)` takes the baseline it is measured against, and only a real disagreement carries an attr. A side within `SAME_CM` (0.005cm) of that baseline *is* the baseline, or Word's 108 twips (0.1905cm) and LibreOffice's 0.191cm would tag every table. Rendered as one `--cell-pad` custom property (the attr's `renderHTML` for the static paths, `TableView` for the node view, since it bypasses `renderHTML`) that `td/th { padding: var(--cell-pad, 0 0.19cm) }` reads — the property inherits, so a cell's own declaration wins for its own `td`. **DOCX**: `w:tblPr/w:tblCellMar` direct, else the table style's (`DocxStyles.tableCellMar` walks `w:basedOn`), and `w:tcPr/w:tcMar` per cell (`cellMarginsCm` reads either, `w:type="nil"` being an explicit zero); export writes `Table.margins` + `TableCell.margins`. **ODF** has no table-level cell margin — it sits on each cell's style, so import takes the first cell's `fo:padding*` for the table and measures the rest against it, and export rides each cell's value on it as a CSS-style TRBL shorthand that `expandCellPadding` splits per side (odf-kit only writes the `fo:padding` shorthand, and ODF allows it only one length). Getting this wrong is not cosmetic: a 0.71cm bibliography marker column lost 0.38cm to the old constant and wrapped `"[2] "` onto three lines.
 - **`tableHeaderRow.ts`** (`TableHeaderRow`) — "header row" as a **styling preset** (not the structural ODF header that repeats per page, by design): `toggleHeaderRowStyle` sets/clears a **light-grey fill (`HEADER_SHADE` `#F2F2F2`)** on the first row's cells; `isHeaderStyled(state, axis)` drives the toolbar toggle's active state. **A table carrying a table style owns these two areas through its Table Style Options instead**: there `toggleHeaderStyle`/`isHeaderStyled` read and write `headerRow`/`firstColumn` in the table's `tableLook` (delegating to `setTableLook`), so the toolbar button and the gallery's checkbox are one state and can't disagree. Both also read their label from the same key (`styles.regions.*`), so they can't drift apart in wording either — the button used to say "header column" where the checkbox said "first column". The fill is the *marker* and **round-trips natively** as `fo:background-color`, so the toggle state survives reopen with no `table:table-header-rows` (⇒ Word/LibreOffice don't repeat it). **Bold is presentational, exactly like a heading**: `tableCellBackground.ts` tags a header-shaded cell with `class="cell-header"` and `editor.css` renders `.cell-header` bold — so *all* its text is bold (existing, typed, pasted, IME) without storing marks, and it stays **editable** via the `fontWeight:'normal'` override (the `Toolbar.svelte` Bold button treats `isInHeaderCell` like a heading: toggles `setFontWeight('normal')`/`unsetFontWeight`). Export (`export/odt.ts`) **bakes** bold onto header-shaded cells' runs (`applyRuns(..., forceBold)`, skipping `fontWeight:'normal'` runs) so Word/LibreOffice match; import (`import/odt.ts`) converts those cells' runs with `boldByDefault` (bold run → no mark, normal run → `fontWeight:'normal'`), mirroring headings. `TableToolbar.svelte` exposes the toggle button.
 
 - **`tableStyle.ts`** (`TableStyle`) — the **table style** family's editor half (model in `styles/tableStyles.ts`, see below): global attrs `tableStyle` on `table` (⇄ `data-table-style`) and `region` on `tableCell`/`tableHeader` (⇄ `data-region`, a space-separated region list). `setTableStyle(name|null)` writes the attr and **materializes** the style into the cells in one transaction — `paintTable` walks the `TableMap`, resolves each cell via `resolveTableCell` (merged cells by their top-left `findCell` box) and writes `backgroundColor`/the four border attrs/`region`, so **export, PDF and clipboard need no new code**. It only writes cells that differ, which makes it idempotent — the condition the re-band plugin depends on. That plugin (`appendTransaction`) repaints a styled table when its row/column **shape** changes (Word's re-banding); deliberately *not* on every transaction, or a manually shaded cell would be repainted away immediately. `refreshTableStyles()` repaints every styled table and is called from `Editor.svelte`'s document-stylesheet `$effect` — the choke point every registry change (edit, rename, reset, import) passes through. `activeTableStyle`/`cellRegionText` feed the gallery and the Bold button (a style's bold region behaves like a heading, see `tableHeaderRow.ts`). `TableView` sets `data-table-style` itself, since the node view bypasses `renderHTML`.