feat(editor): multi-cursor editing MVP — select all occurrences + type into all (#196)

Variant A of #196: VS Code-style multi-cursor limited to "select all
occurrences of a word (or the selection) and type into all at once", built on
top of the existing SearchAndReplace mass-transaction machinery.

- New `MultiCursor` Tiptap extension (packages/editor-ext/src/lib/multi-cursor/):
  Cmd/Ctrl+Shift+L selects all occurrences, Cmd/Ctrl+D adds the next, typing /
  Backspace / Delete apply to every cursor in ONE reverse-order transaction (so a
  single undo reverts the whole multi-edit), Esc / click / navigation collapse.
- Cursors live in plugin state and are remapped on every docChanged — covering
  remote Yjs edits (applied as ordinary transactions) with no collab-specific code.
- Extracted a shared `findOccurrences` util so SearchAndReplace and MultiCursor
  no longer duplicate the occurrence walk (behaviour-preserving).
- Conscious v1 out-of-scope boundaries (Variant B) documented in the extension.

Registered in mainExtensions; carets styled distinctly from collaborative carets.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/editor/extensions/extensions.ts

@@ -45,6 +45,7 @@ import {
   TiptapPdf,
   PageBreak,
   SearchAndReplace,
+  MultiCursor,
   Mention,
   TableDndExtension,
   TableHandleCommandsExtension,
@@ -447,6 +448,10 @@ export const mainExtensions = [
       };
     },
   }).configure(),
+  // Multi-cursor editing (MVP / Variant A): select-all-occurrences + type into
+  // all at once. Does not depend on collaboration, so it lives in mainExtensions
+  // (available in both the plain and collaborative editors).
+  MultiCursor,
   Columns,
   Column,
   AutoJoiner.configure({