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/utils/markdown.ts

@@ -1,4 +1,4 @@
-import { marked } from "marked";
+import { marked, type RendererThis, type Tokens } from "marked";
 import DOMPurify from "dompurify";
 import { linkifyTags, linkifyWikilinks } from "@/utils/tags";
 
@@ -28,7 +28,8 @@ export function stripFirstLineTags(text: string): string {
 }
 
 const headingRenderer = {
-  heading({ text, depth }: { text: string; depth: number }): string {
+  heading(this: RendererThis, { tokens, depth }: Tokens.Heading): string {
+    const text = this.parser.parseInline(tokens);
     const id = slugify(text);
     return `<h${depth} id="${id}">${text}</h${depth}>`;
   },