feat(notes): rich-text editor with tiptap (task 85)

Replace plain textarea with shared tiptap editor and sanitized html rendering.

Adds interactive checklists on read surfaces and mobile overflow fixes.

src/components/rich-text/toggle-checklist.ts

@@ -0,0 +1,28 @@
+const TASK_ITEM_RE = /<li([^>]*data-type="taskItem"[^>]*)>/gi;
+
+export function toggleTaskItemInHtml(html: string, taskIndex: number, checked: boolean): string {
+  let index = 0;
+  return html.replace(TASK_ITEM_RE, (match, attrs: string) => {
+    const current = index;
+    index += 1;
+    if (current !== taskIndex) return match;
+
+    const checkedValue = checked ? "true" : "false";
+    let nextAttrs = attrs;
+
+    if (/data-checked="(?:true|false)"/i.test(nextAttrs)) {
+      nextAttrs = nextAttrs.replace(
+        /data-checked="(?:true|false)"/i,
+        `data-checked="${checkedValue}"`,
+      );
+    } else {
+      nextAttrs = `${nextAttrs} data-checked="${checkedValue}"`;
+    }
+
+    return `<li${nextAttrs}>`;
+  });
+}
+
+export function countTaskItems(html: string): number {
+  return [...html.matchAll(TASK_ITEM_RE)].length;
+}