Refactor agent architecture and add session storage

Major architectural improvements:
- Renamed AgentSession → Agent (state/ → agent/)
- Removed id field from AgentState
- Fixed transport abstraction to pass messages directly instead of using callbacks
- Eliminated circular dependencies in transport creation

Transport changes:
- Changed signature: run(messages, userMessage, config, signal)
- Removed getMessages callback from ProviderTransport and AppTransport
- Transports now filter attachments internally

Session storage:
- Added SessionRepository with IndexedDB backend
- Auto-save sessions after first exchange
- Auto-generate titles from first user message
- Session list dialog with search and delete
- Persistent storage permission dialog
- Browser extension now auto-loads last session

UI improvements:
- ChatPanel creates single AgentInterface instance in setAgent()
- Added drag & drop file upload to MessageEditor
- Fixed artifacts panel auto-opening on session load
- Added "Drop files here" i18n strings
- Changed "Continue Without Saving" → "Continue Anyway"

Web example:
- Complete rewrite of main.ts with clean architecture
- Added check script to package.json
- Session management with URL state
- Editable session titles

Browser extension:
- Added full session storage support
- History and new session buttons
- Auto-load most recent session on open
- Session titles in header

packages/web-ui/example/package.json

@@ -5,7 +5,8 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "check": "tsc --noEmit"
   },
   "dependencies": {
     "@mariozechner/mini-lit": "^0.1.7",