Add “Continue as Guest” UX, Token Handling, and Upgrade Flow

## Summary
Add a Guest Mode to the TPEN-Interfaces (plain JS/HTML/CSS) that:
- Lets users start as guest and obtain a temporary `idToken` from TPEN-Services.
- Uses that token to perform authenticated-like actions in the UI (save transcription, annotate, comment).
- Presents clear limits and a non-intrusive upgrade path (create auth0 user and migrate contributions to them).
- Ensures continuity via refresh token (cookie) and handles token rotation.

## UX Requirements
- Entry Points:
- “Continue as Guest” CTA on the editor or first write attempt.
- Explain constraints: “Guest work may be temporary (2 days of inactivity). Upgrade within 30 days to keep it.”
- Badging & Status:
- Show “Guest” badge in navbar/profile menu with display name, some clearly generated pseudo Latin or Greek author name.
- Tooltip or small info panel with limits (rate, quotas, retention).
- Nudges (Soft Prompts):
- After N actions (configurable), prompt: “Save work across devices—add email (no password).”
- Before share/export: prompt upgrade (if policy requires).
- Upgrade Flow:
- Magic link modal: collect email → call `/guest/claim` → show “Check your email.”
- On claim-complete redirect, call `/guest/upgrade` → swap to user session; update UI.
- Alternate: “Sign in with SSO” button that initiates OIDC flow.
- Export:
- Allow guests to export their current transcription/annotation data as JSON (and/or CSV) without upgrading.

## Frontend Architecture (Plain JS)
- Token Store:
- Keep `idToken` in memory (JS variable).
- Store refresh token in secure HttpOnly cookie (via services response).
- Implement auto-refresh before `idToken` expiry; handle rotation & failure gracefully.
- API Client:
- A small wrapper that attaches `Authorization: Bearer ` to write endpoints.
- On 401 with “token expired”, trigger refresh; if refresh fails, prompt re-start guest or sign in.
- State & UI:
- `authState = { role: 'guest' | 'user', sub, displayName }`.
- Render conditional UI for guest capabilities and limits.

## UI Copy (Suggested)
- Button: Continue as Guest
- Banner: “You’re in Guest Mode. Your work is saved to this device and may expire after 90 days. Upgrade to keep it across devices.”
- Nudge: “Add email to keep your progress.”
- Upgrade Modal:
- Title: “Upgrade your Guest Account”
- Subtext: “We’ll send you a one-click link. No password needed.”
- Field: Email
- CTA: Send Magic Link
- Success: “Check your email—click the link to finish.”
- Post-upgrade: “All your work has been moved to your account.”

## API Integrations
- Start Guest: `POST /guest/start` → receive `idToken`, `refreshToken`, `displayName`, `policy`.
- Refresh: `POST /auth/refresh` (automatic before expiry).
- Write Ops: existing endpoints (transcriptions, annotations, comments) with guest `idToken`.
- Claim/Upgrade: `POST /guest/claim` (email); `POST /guest/upgrade` on redirect.

## Error Handling
- Show clear messages on:
- Project policy denial (`allowGuestEdits=false`): “This project does not allow guest edits. Sign in to continue.”
- Quota exceeded: “Guest quota reached. Upgrade to continue editing.”
- Rate limit triggered: “Too many actions. Please wait a moment.”
- Refresh failure: “Session expired. Continue as Guest or Sign in.”

## Accessibility & i18n
- Keep guest badge and prompts accessible (ARIA).
- Prepare strings for translation; avoid hard-coded English.

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## Tasks (Checklist)
- [ ] Add “Continue as Guest” button and entry UX.
- [ ] Implement start-guest call (`/guest/start`) and token storage logic.
- [ ] Implement auto-refresh flow (pre-expiry) with cookie-based refresh.
- [ ] Add guest badge and info panel with limits/retention messaging.
- [ ] Gate UI actions based on `role=guest` vs `role=user` and project policy responses.
- [ ] Implement upgrade modal (email input) → call `/guest/claim`.
- [ ] Handle claim-complete redirect → call `/guest/upgrade` → update session/UI.
- [ ] Add export for guest data (JSON/CSV, depending on current features).
- [ ] Add error/limit banners and retry affordances.
- [ ] Telemetry: track guest session starts, write volumes, upgrade conversions.
- [ ] Update README/help docs with Guest Mode explanation.

## Acceptance Criteria
- Users can enter Guest Mode and perform allowed actions.
- Token rotation works seamlessly; the UI stays authenticated as guest.
- Project policy denials are surfaced clearly; allowed actions function normally.
- Upgrade via magic link flows correctly and preserves the user’s work.
- Export works for guests without requiring upgrade.
- Strings and UI are accessible and translatable (where applicable).
- Documentation and help content updated.

## Open Questions
- Exact copy for retention.
- Which endpoints are blocked for guests (invite/share/private project settings)?