feat: add session history sidebar with delete and navigation

- SessionSidebar component listing past sessions sorted by recency
- Session title preview and relative date display
- Click to switch sessions, delete with confirmation
- Added deleteSession method to session store
- Integrated sidebar into chat page layout

Closes #12

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/stores/sessions.svelte.ts

@@ -91,6 +91,16 @@ function createSessionStore() {
 		}
 	}
 
+	function deleteSession(id: string) {
+		sessions.delete(id);
+		if (activeSessionId === id) {
+			const remaining = getAllSessions();
+			activeSessionId = remaining.length > 0 ? remaining[0].id : null;
+			if (activeSessionId) saveActiveSessionId(activeSessionId);
+		}
+		saveSessions(sessions);
+	}
+
 	function getAllSessions(): Session[] {
 		return [...sessions.values()].sort((a, b) => b.createdAt.getTime() - a.createdAt.getTime());
 	}
@@ -102,6 +112,7 @@ function createSessionStore() {
 		getOrCreateSession,
 		updateMessages,
 		switchSession,
+		deleteSession,
 		getAllSessions
 	};
 }