fix(playground): client-side session loading

[devallibus]

Summary

• Replace createServerFn/useServerFn with direct API calls for session loading
• The playground doesn't need SSR — editor + canvas are purely client-side
• Eliminates hydration timing issues where useSearch reactive values aren't available when onMount fires
• Remove auth requirement from POST /api/playground/create (creating an empty session is harmless — the UUID is the capability token)

What was wrong

1. createResource doesn't integrate with TanStack Router SSR streaming → blank page
2. useServerFn + onMount worked but useSearch reactive value isn't hydrated at mount time → session ID always undefined → MCP-created sessions ignored, new session created instead
3. Server function serialization (seroval) was sending undefined for the session ID

How it works now

• onMount reads ?session= from window.location.search directly
• Calls /api/playground/:id/state to load existing session, or /api/playground/create for new ones
• SSE push from MCP → browser already works (unchanged)

Test plan

• Visit /playground → new session created, editor + canvas render
• MCP create_playground → get session URL → open in browser → same session loads (not a new one)
• MCP update_shader with browser open → canvas updates live via SSE
• MCP get_errors with broken GLSL → returns compilation errors

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