[NRT-822] Qt5: open Terms & Conditions and Flashcard Selector in the external browser

[RisingOrange]

What & why

On Qt5 builds of Anki the embedded webview is Qt WebEngine 5.x (Chromium 77), which can't run the modern JS used by two AnkiHub pages we render in-app, so they show up blank/broken for these users:

• Terms & Conditions — a sync-blocker: a user asked to accept (updated) terms can't, so they can't sync or use AnkiHub at all.
• Flashcard Selector — non-functional.

These are old-macOS (10.13/10.14) users who can't upgrade to Qt6 Anki yet meet min_anki_version, so it's a supported config. Refs: TRIAGE-147, ACTV-94; Sentry JAVASCRIPT-18 / JAVASCRIPT-E / JAVASCRIPT-4 (Qt Web Engine 5.14.2).

Approach

Gate both surfaces on using_qt5() and open them in the user's external (modern) browser instead of the dead embedded engine — mirroring how the chatbot is Qt6-gated. No backend changes.

• Flashcard Selector (flashcard_selector_dialog.py) — on Qt5, openLink(url_flashcard_selector(ah_did)) + a short toast instead of building the embedded dialog.
• Terms & Conditions (terms_dialog.py) — on Qt5, open the terms page externally and show a new ExternalBrowserTermsDialog that detects acceptance:
  • bounded auto-poll (every 3s, up to 3min), then stops; a manual "Check now" button remains as the fallback. Both share the same check + resume logic.
  • on detection → close dialog and resume the blocked sync (errors.py).
• Acceptance check — new AnkiHubClient.is_terms_agreement_accepted() re-issues a request guarded by HasFilledPersonalInfo (GET /users/decks/): 200 = accepted, 403 = not yet. Verified against the webapp: the terms page sets all gated fields (terms + country + study prefs) atomically, so the 403→200 flip maps exactly onto "user completed the page".

Status / open question

The Jira issue is in Ready for Design — the ExternalBrowserTermsDialog UX (auto-poll vs. manual-only, copy, whether it blocks Anki) is pending a designer's review. This PR is a draft implementation/reference, not final; the Terms dialog UI may change after design.

Test plan

• Qt5 Anki: Terms page + Flashcard Selector open in the system browser and work.
• After accepting terms in the browser, the add-on auto-detects (within the poll window) or via "Check now", closes the dialog, and sync proceeds.
• Qt6 Anki: unchanged (embedded dialogs still used).
• Verify on macOS (per repo policy for UI changes).

https://claude.ai/code/session_01E1qJDe6g6ea63Fusg9fViv