feat: deliver P2 v0.2.0 with unified editor, search, and snapshots

Implement schema v2 migration, outline/setting/inspiration CRUD, unified TipTap editing, reference panel with mentions, FTS global search, chapter version history, writing landmarks, word frequency analysis, light theme contrast fixes, and full unit/E2E test coverage.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/renderer/extensions/landmark-mark.ts

@@ -0,0 +1,27 @@
+import { Mark, mergeAttributes } from '@tiptap/core'
+
+export const LandmarkMark = Mark.create({
+  name: 'landmark',
+  inclusive: true,
+  addAttributes() {
+    return {
+      label: { default: '' },
+      landmarkType: { default: 'todo' }
+    }
+  },
+  parseHTML() {
+    return [{ tag: 'span[data-landmark]' }]
+  },
+  renderHTML({ HTMLAttributes }) {
+    const label = HTMLAttributes.label ?? ''
+    return [
+      'span',
+      mergeAttributes(HTMLAttributes, {
+        'data-landmark': 'true',
+        class: 'landmark-mark',
+        'data-testid': 'landmark-mark'
+      }),
+      `[TODO: ${label}]`
+    ]
+  }
+})