[Agent] Embed form submittable before anonymous auth completes

Agent brief

Category: bug | Severity: high | Effort: S

Problem

Embed authState starts as 'pending' but the submit form renders immediately. User can submit before signInAnonymously() completes → unauthenticated error.

Repro

1. Open embed on anonymous project
2. Type title quickly and submit before auth resolves
3. Error: "Please reload and try again"

Files

• src/app/embed/[projectSlug]/client.tsx — add pending loading state

Acceptance criteria

• Show spinner/skeleton while authState === 'pending'
• Submit disabled until authState === 'ok'
• No race on fast submit

Suggested approach

Early return loading UI for pending; disable submit button until ok.