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/utils/markdown.ts

@@ -38,7 +38,7 @@ const headingRenderer = {
 marked.use({ renderer: headingRenderer });
 
 const PURIFY_OPTS_FULL = {
-  ADD_ATTR: ["data-tag", "data-title"],
+  ADD_ATTR: ["data-tag", "data-title", "data-task-index"],
   FORCE_BODY: true,
 };
 
@@ -47,10 +47,22 @@ const PURIFY_OPTS_PREVIEW = {
   FORCE_BODY: true,
 };
 
-export function renderMarkdown(text: string): string {
+export function renderMarkdown(text: string, { interactiveCheckboxes = false } = {}): string {
   const stripped = stripFirstLineTags(text);
   const decoded = decodeEntities(stripped);
-  const html = marked(decoded) as string;
+  let html = marked(decoded) as string;
+
+  if (interactiveCheckboxes) {
+    // marked renders task-list checkboxes as <input ... disabled="" ...>.
+    // Remove disabled and stamp a sequential data-task-index so the viewer
+    // can identify which item was toggled regardless of attribute order.
+    html = html.replace(/ disabled=""/g, "");
+    let taskIdx = 0;
+    html = html.replace(/<input ([^>]*type="checkbox"[^>]*)>/g, (_m, attrs) => {
+      return `<input data-task-index="${taskIdx++}" ${attrs}>`;
+    });
+  }
+
   const withTags = linkifyTags(html);
   const withLinks = linkifyWikilinks(withTags);
   const sanitized = DOMPurify.sanitize(withLinks, PURIFY_OPTS_FULL);