feat: the table's four row/column inserts are big buttons

Rows & Columns follows Word: four big buttons carrying a 28px glyph and a
two-line label, with the three deletions folded into one Entfernen dropdown
beside them so the group keeps its width.

Two lines cost the band 12px (--w-ribbon-h 96 -> 108). The labels size to
min-content, not max-width, which a wrapped box ignores; and the inserts drop
the reserved caret slot, without which the group's foot label sits 8px below
every other group's.

docs/architecture/ribbon.md

@@ -46,7 +46,7 @@ so the split is invisible there.
 Ribbon.svelte
 ├─ .ribbon-tabs     File pill · quick access (save, undo, redo) · tabs · contextual tabs
 │                   · spacer · document name · appearance · UI language
-└─ .ribbon-body     the active tab's groups, a fixed --w-ribbon-h (96px) band
+└─ .ribbon-body     the active tab's groups, a fixed --w-ribbon-h (108px) band
 ```
 
 The band's height never changes: it scrolls horizontally with a hidden scrollbar instead of