Add custom message extension system with typed renderers and message transformer

- Implement CustomMessages interface for type-safe message extension via declaration merging
- Add MessageRenderer<T> with generic typing for custom message rendering
- Add messageTransformer to Agent for filtering/transforming messages before LLM
- Move message filtering from transports to Agent (separation of concerns)
- Add message renderer registry with typed role support
- Update web-ui example with SystemNotificationMessage demo
- Add custom transformer that converts notifications to <system> tags
- Add SessionListDialog onDelete callback for active session cleanup
- Handle non-existent session IDs in URL (redirect to new session)
- Update both web-ui example and browser extension with session fixes

packages/web-ui/src/dialogs/SessionListDialog.ts

@@ -11,13 +11,15 @@ export class SessionListDialog extends DialogBase {
 	@state() private loading = true;
 
 	private onSelectCallback?: (sessionId: string) => void;
+	private onDeleteCallback?: (sessionId: string) => void;
 
 	protected modalWidth = "min(600px, 90vw)";
 	protected modalHeight = "min(700px, 90vh)";
 
-	static async open(onSelect: (sessionId: string) => void) {
+	static async open(onSelect: (sessionId: string) => void, onDelete?: (sessionId: string) => void) {
 		const dialog = new SessionListDialog();
 		dialog.onSelectCallback = onSelect;
+		dialog.onDeleteCallback = onDelete;
 		dialog.open();
 		await dialog.loadSessions();
 	}
@@ -54,6 +56,11 @@ export class SessionListDialog extends DialogBase {
 
 			await storage.sessions.deleteSession(sessionId);
 			await this.loadSessions();
+
+			// Notify callback that session was deleted
+			if (this.onDeleteCallback) {
+				this.onDeleteCallback(sessionId);
+			}
 		} catch (err) {
 			console.error("Failed to delete session:", err);
 		}