feat: placeholder fields (inline atom + ODF/DOCX round trip)

An inline atom drawn gray as ‹label›: a click selects the whole field,
typing replaces it, Tab/Shift-Tab jump between fields. Exports to ODF
<text:placeholder> (PLH sentinel U+E022) and a DOCX w:sdt content
control tagged edentext-placeholder; both importers read it back, the
export-minted gray stripped. Covered in roundtrip Leg 31 + fuzzDoc.

src/styles/editor.css

@@ -885,6 +885,16 @@
   height: 0;
 }
 
+/* A template placeholder field: gray until replaced, as LibreOffice draws its own.
+   The sheet keeps its own colors through every theme, so the gray is fixed too. */
+.paper .tiptap span[data-placeholder-field] {
+  color: #808080;
+}
+.paper .tiptap span[data-placeholder-field].ProseMirror-selectednode {
+  background: color-mix(in srgb, var(--color-primary) 25%, transparent);
+  outline: none;
+}
+
 /* Paragraph mark (¶). Out of flow at its static position — after the last character,
    but invisible to line breaking, so a line-end space still hangs into the margin and
    the mark can never be pushed onto a line of its own. The toggle only recolors it. */