feat: SvelteKit SSR support with server-side XRPC, OAuth sessions, and dev tooling
Rewrite server.ts to Request→Response handler, add SSR renderer and
Node.js adapter, implement session cookies for viewer resolution in
SSR, add /__dev/login endpoint for test auth, register core XRPC
handlers for preferences, fix OAuth scope joining, switch test context
to SQLite, and add /__dev/ route forwarding in vite plugin.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>