feat: Frontend platform linking page (/link/{token})

Adds the user-facing page that completes the platform bot linking flow.

When an unlinked user messages the bot, they get a URL like:
  https://platform.agpt.co/link/{token}

This page:
1. Validates the token (expired? already used?)
2. If user isn't logged in → redirects to login with ?next=/link/{token}
3. Shows a confirmation screen: 'Link your [platform] account to AutoGPT'
4. On click → calls POST /api/platform-linking/tokens/{token}/confirm
5. Shows success or error state

## Implementation
- Lives in (no-navbar) route group (standalone page, no main nav)
- Reuses AuthCard, Button, Text, Link components from existing auth pages
- Same visual style as login/signup pages
- Handles all edge cases: expired token, already linked, not authenticated

## Stacked on
- feat/copilot-bot-service (PR #12618)
- feat/platform-bot-linking (PR #12615)