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/lib/adapters/file-bridge.ts

@@ -0,0 +1,29 @@
+import { invoke } from '@tauri-apps/api/core';
+import { listen, type UnlistenFn } from '@tauri-apps/api/event';
+
+export interface FileChangedPayload {
+  pane_id: string;
+  path: string;
+  content: string;
+}
+
+/** Start watching a file; returns initial content */
+export async function watchFile(paneId: string, path: string): Promise<string> {
+  return invoke('file_watch', { paneId, path });
+}
+
+export async function unwatchFile(paneId: string): Promise<void> {
+  return invoke('file_unwatch', { paneId });
+}
+
+export async function readFile(path: string): Promise<string> {
+  return invoke('file_read', { path });
+}
+
+export async function onFileChanged(
+  callback: (payload: FileChangedPayload) => void
+): Promise<UnlistenFn> {
+  return listen<FileChangedPayload>('file-changed', (event) => {
+    callback(event.payload);
+  });
+}

v2/src/lib/adapters/session-bridge.ts

@@ -0,0 +1,45 @@
+import { invoke } from '@tauri-apps/api/core';
+
+export interface PersistedSession {
+  id: string;
+  type: string;
+  title: string;
+  shell?: string;
+  cwd?: string;
+  args?: string[];
+  created_at: number;
+  last_used_at: number;
+}
+
+export interface PersistedLayout {
+  preset: string;
+  pane_ids: string[];
+}
+
+export async function listSessions(): Promise<PersistedSession[]> {
+  return invoke('session_list');
+}
+
+export async function saveSession(session: PersistedSession): Promise<void> {
+  return invoke('session_save', { session });
+}
+
+export async function deleteSession(id: string): Promise<void> {
+  return invoke('session_delete', { id });
+}
+
+export async function updateSessionTitle(id: string, title: string): Promise<void> {
+  return invoke('session_update_title', { id, title });
+}
+
+export async function touchSession(id: string): Promise<void> {
+  return invoke('session_touch', { id });
+}
+
+export async function saveLayout(layout: PersistedLayout): Promise<void> {
+  return invoke('layout_save', { layout });
+}
+
+export async function loadLayout(): Promise<PersistedLayout> {
+  return invoke('layout_load');
+}