feat(v2): implement session persistence, file watcher, and markdown viewer

Phase 4 complete (MVP ship):
- SessionDb (rusqlite, WAL mode): sessions + layout_state tables, CRUD
- FileWatcherManager (notify v6): watch files, emit Tauri change events
- MarkdownPane: marked.js rendering with Catppuccin styles, live reload
- Layout store wired to persistence (addPane/removePane/setPreset persist)
- restoreFromDb() on startup restores panes in layout order
- Sidebar "M" button opens file picker for markdown files
- New adapters: session-bridge.ts, file-bridge.ts
- Deps: rusqlite (bundled), dirs 5, notify 6, marked

v2/package.json

@@ -25,6 +25,7 @@
     "@tauri-apps/api": "^2.10.1",
     "@xterm/addon-canvas": "^0.7.0",
     "@xterm/addon-fit": "^0.11.0",
-    "@xterm/xterm": "^6.0.0"
+    "@xterm/xterm": "^6.0.0",
+    "marked": "^17.0.4"
   }
 }