feat: interactive checkboxes in list note viewer

- renderMarkdown() accepts interactiveCheckboxes option: removes disabled=""
  and stamps data-task-index on each checkbox in the marked HTML output
- NoteViewerView detects list notes by body content (- [ ] / - [x] pattern)
  and passes interactiveCheckboxes: true when rendering
- onBodyChange() handles checkbox change events: toggles the matching line
  in the body, optimistically updates the store, then PATCHes the note
- prose.css adds .prose--checklist rules for marked output: no bullet,
  flex row, accent-color, line-through on checked items via :has()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/assets/prose.css

@@ -180,6 +180,34 @@
   color: var(--color-text-muted);
 }
 
+/* Interactive checkboxes — marked output in the list-note viewer */
+.prose--checklist ul {
+  list-style: none;
+  padding-left: 0.25rem;
+}
+.prose--checklist li {
+  display: flex;
+  align-items: baseline;
+  gap: 0.5rem;
+  margin-bottom: 0.25rem;
+}
+.prose--checklist li input[type="checkbox"] {
+  flex-shrink: 0;
+  accent-color: var(--color-primary);
+  cursor: pointer;
+  width: 0.95em;
+  height: 0.95em;
+  margin: 0;
+}
+.prose--checklist li:has(input[type="checkbox"]:checked) > p,
+.prose--checklist li:has(input[type="checkbox"]:checked) {
+  text-decoration: line-through;
+  color: var(--color-text-muted);
+}
+.prose--checklist li:has(input[type="checkbox"]:checked) input[type="checkbox"] {
+  text-decoration: none; /* don't strike through the checkbox itself */
+}
+
 /* Tiptap editor */
 .tiptap-editor .ProseMirror {
   outline: none;