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/components/TiptapEditor.vue

@@ -3,12 +3,11 @@ import { watch, onBeforeUnmount } from "vue";
 import { Editor, EditorContent } from "@tiptap/vue-3";
 import StarterKit from "@tiptap/starter-kit";
 import Link from "@tiptap/extension-link";
-import Placeholder from "@tiptap/extension-placeholder";
+import { Placeholder } from "@tiptap/extensions";
 import { marked } from "marked";
 import DOMPurify from "dompurify";
 import { serializeToMarkdown } from "@/utils/markdownSerializer";
-import TaskList from "@tiptap/extension-task-list";
-import TaskItem from "@tiptap/extension-task-item";
+import { TaskList, TaskItem } from "@tiptap/extension-list";
 import { TagDecoration } from "@/extensions/TagDecoration";
 import { WikilinkDecoration } from "@/extensions/WikilinkDecoration";
 import { WikilinkSuggestion } from "@/extensions/WikilinkSuggestion";
@@ -71,6 +70,7 @@ try {
     extensions: [
       StarterKit.configure({
         heading: { levels: [1, 2, 3, 4, 5, 6] },
+        link: false,
       }),
       Link.configure({
         openOnClick: false,