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({

apps/client/src/features/editor/styles/index.css

@@ -1,5 +1,6 @@
 @import "./core.css";
 @import "./collaboration.css";
+@import "./multi-cursor.css";
 @import "./task-list.css";
 @import "./placeholder.css";
 @import "./drag-handle.css";

apps/client/src/features/editor/styles/multi-cursor.css

@@ -0,0 +1,60 @@
+/*
+ * Multi-cursor (issue #196). Deliberately DISTINCT from the collaboration
+ * carets (collaboration.css) so a user never confuses their own multi-cursors
+ * with a co-author's caret: solid accent-blue carets + a translucent blue
+ * range highlight, versus the thin dark collaboration caret with a name label.
+ */
+
+/* A secondary caret rendered as a Decoration.widget at each cursor position. */
+.multi-cursor__caret {
+    position: relative;
+    display: inline-block;
+    width: 0;
+    height: 1em;
+    vertical-align: text-bottom;
+    pointer-events: none;
+}
+
+.multi-cursor__caret::after {
+    content: "";
+    position: absolute;
+    left: -1px;
+    top: 0;
+    bottom: 0;
+    width: 2px;
+    background: #2b6cb0;
+    animation: multi-cursor-blink 1s steps(1) infinite;
+}
+
+/* Optional label class reserved for future per-cursor annotations. */
+.multi-cursor__label {
+    position: absolute;
+    top: -1.4em;
+    left: -1px;
+    font-size: 0.7rem;
+    line-height: normal;
+    padding: 0.05rem 0.25rem;
+    border-radius: 3px 3px 3px 0;
+    background: #2b6cb0;
+    color: #fff;
+    white-space: nowrap;
+    user-select: none;
+    pointer-events: none;
+}
+
+/* Inline highlight for a multi-cursor RANGE (from < to). */
+.multi-cursor__selection {
+    background: rgba(43, 108, 176, 0.28);
+    border-radius: 2px;
+}
+
+@keyframes multi-cursor-blink {
+    0%,
+    50% {
+        opacity: 1;
+    }
+    51%,
+    100% {
+        opacity: 0;
+    }
+}