Linting, tests, README, CI, and code quality improvements

Linting:
- analysis_options.yaml: flutter_lints base + 20 additional rules
  (prefer_single_quotes, avoid_print, cancel_subscriptions,
  unawaited_futures, empty_catches, always_declare_return_types, …)
- unused_import / dead_code treated as errors

pubspec.yaml:
- Remove 6 unused packages: freezed, freezed_annotation, json_annotation,
  json_serializable, riverpod_generator, collection
- Add test: ^1.25.0 for pure-Dart unit tests

Testable utilities (extracted from screen code):
- lib/core/utils/html_utils.dart — htmlToPlain()
- lib/core/utils/format_utils.dart — fmtSize()
- lib/core/utils/logger.dart — thin debugPrint wrapper

Unit tests (test/unit/, no device required):
- html_utils_test.dart — 13 cases covering tags, entities, edge cases
- format_utils_test.dart — B / KB / MB / GB formatting
- email_model_test.dart — EmailAddress JSON roundtrip, toString, EmailDraft
- account_sync_backoff_test.dart — exponential backoff + clamping

Taskfile:
- task test → dart test test/unit/ (fast, no device)
- task test-flutter → flutter test (widget tests)
- task check → analyze + unit tests in parallel

README rewrite:
- Covers user flow (download, add account)
- Developer flow: nix develop, direnv allow, task codegen, task check
- Platform status table (Linux done, others pending)
- Project layout diagram
- Schema-change workflow

CI (.github/workflows/ci.yml):
- analyze-and-test job: flutter analyze + dart test on every push/PR
- integration job: Nix + Stalwart on push to main
- build-linux job: flutter build linux --release

Logging:
- Replace all bare catch (_) {} with log() calls
- Sync backoff errors now print account email + retry delay
- STATUS failures on \Noselect mailboxes logged at debug level
- STARTTLS failures logged before continuing without TLS

.gitignore:
- Add .direnv/, .flutter-plugins-dependencies
- Add all platform generated-plugin wiring files
- Add .env / .env.local
- Add linux/build/

.env.example: documents optional STALWART_PORT overrides

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

pubspec.yaml

@@ -23,7 +23,6 @@ dependencies:
 
   # State management
   flutter_riverpod: ^2.6.1
-  riverpod_annotation: ^2.4.1
 
   # Navigation
   go_router: ^14.8.1
@@ -31,21 +30,16 @@ dependencies:
   # Secure credential storage (passwords)
   flutter_secure_storage: ^9.2.4
 
-  # Utilities
-  freezed_annotation: ^2.4.4
-  json_annotation: ^4.9.0
+  # Date formatting
   intl: any
-  collection: ^1.19.1
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
   flutter_lints: ^4.0.0
   drift_dev: ^2.20.3
-  riverpod_generator: ^2.6.2
-  freezed: ^2.5.8
-  json_serializable: ^6.9.0
   build_runner: ^2.4.13
+  test: ^1.25.0
 
 flutter:
   uses-material-design: true