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/src-tauri/Cargo.toml

@@ -24,3 +24,6 @@ tauri = { version = "2.10.3", features = [] }
 tauri-plugin-log = "2"
 portable-pty = "0.8"
 uuid = { version = "1", features = ["v4"] }
+rusqlite = { version = "0.31", features = ["bundled"] }
+dirs = "5"
+notify = { version = "6", features = ["macos_fsevent"] }