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/package.json

@@ -13,7 +13,7 @@
     "clean": "rm -rf dist",
     "build": "tsc -p tsconfig.build.json && tailwindcss -i ./src/app.css -o ./dist/app.css --minify",
     "dev": "concurrently --names \"build,example\" --prefix-colors \"cyan,green\" \"tsc -p tsconfig.build.json --watch\" \"tailwindcss -i ./src/app.css -o ./dist/app.css --watch\" \"npm run dev --prefix example\"",
-    "typecheck": "tsc --noEmit",
+    "typecheck": "tsc --noEmit && cd example && tsc --noEmit",
     "check": "npm run typecheck"
   },
   "dependencies": {