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/routes/chat/+page.ts

@@ -0,0 +1,2 @@
+export const prerender = false;
+export const ssr = false;