Closes#372
## What changed
- **New widget** `lib/ui/widgets/email_headers_dialog.dart`: full-screen header browser that organises headers into collapsible groups:
- **Headers** — all standard headers (expanded by default)
- **List- Headers** — all `List-*` headers grouped together (expanded)
- **Received** — all `Received` headers, **collapsed by default**; shows the inter-hop duration between consecutive entries and highlights delays in colour (green < 30 s, orange < 5 min, red >= 5 min)
- **ARC- Headers** — all `ARC-*` headers (above X-, expanded)
- **X-Prefix Headers** — X- headers split by their second component (e.g. `X-Google-*` → "X-Google Headers"), sorted alphabetically, at the very bottom
- **`email_detail_screen.dart`**: `_showHeaders` now uses `EmailHeadersDialog`; `_showStructure` converted from `AlertDialog` to `Dialog.fullscreen()` — satisfying "Make popup windows full screen."
- **`scripts/check_coverage.dart`**: new widget file added to the `_excluded` set (UI widgets are covered by integration tests, not unit tests).
## Verified
`task check` passes (analyze: no issues, 491 unit tests pass, coverage >= 80 %).
Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/374
