feat(featurebase): switch to JWT auth and restore feedback widget

[johnwschoi]

Summary

• Switches Featurebase widget auth from untrusted client-side identity to backend-signed JWT
• Adds dedicated getFeaturebaseToken tRPC query through the runtime stack (api-contract → SDK boundary → provider service → runtime-api → app-router → browser query helper)
• Preserves the existing Featurebase widget / portal UX; this PR changes auth plumbing only
• Uses a dedicated Featurebase token flow instead of piggybacking on fetch me / getClineAccountProfile, because the JWT is short-lived and Featurebase-specific and should be fetched fresh on widget open rather than carried on a generic profile response
• Rewrites useFeaturebaseFeedbackWidget so it:
  • removes mount-time identify({ email, name, userId })
  • fetches a fresh JWT on each widget open
  • calls Featurebase("identify", { featurebaseJwt })
  • opens the widget only after the identify callback succeeds
• Restores the Share Feedback button and app hook wiring removed by f89668d
• Fails closed on auth/token errors and surfaces toast errors instead of silent no-op behavior
• Gates Share Feedback on an authenticated Cline session and shows a sign-in message when unavailable

Test plan

• Runtime tests: JWT success, retry-after-refresh, no provider throws, non-cline provider throws
• Hook tests:
  • token fetch occurs on open, not mount
  • featurebaseJwt is passed to identify
  • no mount-time identify with plain user metadata
  • widget opens only after identify success callback
  • identify failure shows toast and does not open widget
  • token-load failure shows toast and does not open widget
• FeedbackCard tests:
  • disabled without Cline session
  • sign-in helper message shown when unavailable
  • enabled with authenticated Cline session
• Updated Kanban test coverage passing locally

Dependencies

• Requires backend endpoint: cline/core-platform#2259
• Requires SDK method: feat(sdk): add fetchFeaturebaseToken method [ENG-1673] cline#10005
• Manual follow-up:
  • provision FEATUREBASE_JWT_SECRET
  • enable Featurebase secure login
  • disable anonymous posting
  • configure board privacy as planned

Ref: ENG-1673