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/src/storage/app-storage.ts

@@ -1,5 +1,6 @@
 import { LocalStorageBackend } from "./backends/local-storage-backend.js";
 import { ProviderKeysRepository } from "./repositories/provider-keys-repository.js";
+import { SessionRepository } from "./repositories/session-repository.js";
 import { SettingsRepository } from "./repositories/settings-repository.js";
 import type { AppStorageConfig } from "./types.js";
 
@@ -10,6 +11,7 @@ import type { AppStorageConfig } from "./types.js";
 export class AppStorage {
 	readonly settings: SettingsRepository;
 	readonly providerKeys: ProviderKeysRepository;
+	readonly sessions?: SessionRepository;
 
 	constructor(config: AppStorageConfig = {}) {
 		// Use LocalStorage with prefixes as defaults
@@ -18,6 +20,11 @@ export class AppStorage {
 
 		this.settings = new SettingsRepository(settingsBackend);
 		this.providerKeys = new ProviderKeysRepository(providerKeysBackend);
+
+		// Session storage is optional
+		if (config.sessions) {
+			this.sessions = new SessionRepository(config.sessions);
+		}
 	}
 }