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/App.svelte

@@ -2,7 +2,7 @@
   import { onMount, onDestroy } from 'svelte';
   import SessionList from './lib/components/Sidebar/SessionList.svelte';
   import TilingGrid from './lib/components/Layout/TilingGrid.svelte';
-  import { addPane, focusPaneByIndex, getPanes } from './lib/stores/layout.svelte';
+  import { addPane, focusPaneByIndex, getPanes, restoreFromDb } from './lib/stores/layout.svelte';
   import { startAgentDispatcher, stopAgentDispatcher } from './lib/agent-dispatcher';
 
   function newTerminal() {
@@ -27,6 +27,7 @@
 
   onMount(() => {
     startAgentDispatcher();
+    restoreFromDb();
 
     function handleKeydown(e: KeyboardEvent) {
       // Ctrl+N — new terminal