feat: redesign invitation acceptance flow with inline signup/login and auto-join

[neethika12]

Problem

1. No account — clicking an invitation link with no account shows "Invitation unavailable" instead of a signup prompt
2. No auto-join — after signing up, users aren't automatically added to the workspace
3. Wrong email — no clear message when logged in with a different account than the one invited
4. Email not sent — CreateInvitation was not sending invitation emails
5. SMTP reliability — SMTP goroutine had no timeout or panic recovery
6. Stuck button — invite button never reset after success/failure

Solution

Backend: send invitation email in a safe goroutine with defer recover(), add 15s SMTP timeout, expose email in ValidateInvitation response

Frontend: full state machine on the invitation page — not logged in → inline Sign In / Create Account forms; after signup → auto-login → auto-accept; after sign-in → auto-accept; wrong email logged in → explicit mismatch message

Acceptance Criteria

• User with no account sees inline signup form pre-filled with their email
• After signup, user is automatically added to the workspace
• User with existing account is auto-added after sign-in
• Wrong-email login shows: "You are signed in as X but this invitation was sent to Y"
• Invitation emails are sent on invite
• Invite button resets correctly after success or failure