Upgrade all major frontend dependencies

- TipTap 2 → 3: Extension from @tiptap/core, Placeholder from
  @tiptap/extensions, TaskList/TaskItem from @tiptap/extension-list,
  link: false in StarterKit (now bundles Link), @tiptap/core added
- marked 15 → 17: heading renderer updated to tokens/parseInline API
- Pinia 2 → 3, Vue Router 4 → 5 (no code changes required)
- Vite 6 → 7, @vitejs/plugin-vue 5 → 6, vue-tsc 2 → 3
- TypeScript 5.6 → 5.9: fixed Uint8Array<ArrayBuffer> strictness in
  push.ts, removed unused bodyEl ref in NoteViewerView.vue
- .npmrc: legacy-peer-deps=true for TipTap v3 peer dep resolution

TipTap 3 new capabilities now available: static renderer
(createStaticRenderer), MarkViews, @tiptap/extensions package.

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

frontend/src/views/NoteViewerView.vue

@@ -12,7 +12,6 @@ import TableOfContents from "@/components/TableOfContents.vue";
 const route = useRoute();
 const router = useRouter();
 const store = useNotesStore();
-const bodyEl = ref<HTMLElement | null>(null);
 const backlinks = ref<{ type: string; id: number; title: string }[]>([]);
 const converting = ref(false);
 
@@ -198,7 +197,6 @@ async function convertToTask() {
           />
         </div>
         <div
-          ref="bodyEl"
           class="body prose"
           v-html="renderedBody"
           @click="onBodyClick"