fix: prevent session creation loop when running without backend

Make getOrCreateSession idempotent by returning the existing active
session when called with no ID, instead of always creating a new one.
Use untrack() in the $effect to sever SvelteMap dependency so it only
re-runs on $page changes. Disable SSR on client-only routes, reduce
preload aggressiveness, and skip retries when already disconnected.

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

src/lib/stores/sessions.svelte.ts

@@ -67,6 +67,10 @@ function createSessionStore() {
 			saveActiveSessionId(id);
 			return sessions.get(id)!;
 		}
+		// No specific ID — prefer existing active session
+		if (!id && activeSessionId && sessions.has(activeSessionId)) {
+			return sessions.get(activeSessionId)!;
+		}
 		return createSession(id);
 	}