feat: group email headers in full-screen dialog (#374)

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

scripts/check_coverage.dart

@@ -62,6 +62,7 @@ const _excluded = {
   'lib/ui/screens/about_screen.dart',
   'lib/ui/screens/email_action_helpers.dart',
   'lib/ui/utils/about_markdown.dart',
+  'lib/ui/widgets/email_headers_dialog.dart',
   'lib/ui/widgets/email_tile.dart',
   'lib/core/sync/account_sync_manager.dart',
   'lib/core/sync/background_sync.dart',