Add help responses, claimed tab, claim errors, sheets auth

- Help tab: inline response form (note + URL + annotation layer),
  responses displayed on resolved items with jump links
- Cell Library: new Claimed tab showing all claimed cells with
  claimer display names resolved from Supabase
- Claim errors: visible error banner instead of console-only warning,
  auto-dismisses after 5 seconds
- Notification detail image: max-height increased to 600px
- Google Sheets write-back: service account JWT auth via Web Crypto API
  (replaces broken unauthenticated API calls)
- Supabase: response_note, response_url, response_annotation_layer
  columns on help_requests table

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: amyleesterling