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>

.github/workflows/ci.yml

@@ -0,0 +1,89 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  analyze-and-test:
+    name: Analyze & unit test
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: subosito/flutter-action@v2
+        with:
+          flutter-version: "3.41.6"
+          channel: stable
+          cache: true
+
+      - name: Install dependencies
+        run: flutter pub get
+
+      - name: Generate Drift code
+        run: dart run build_runner build --delete-conflicting-outputs
+
+      - name: Analyze
+        run: flutter analyze --fatal-infos
+
+      - name: Unit tests
+        run: dart test test/unit/
+
+  integration:
+    name: Integration tests (Stalwart)
+    runs-on: ubuntu-latest
+    # Run integration tests only on push to main, not on every PR,
+    # since they require downloading the Stalwart binary via Nix.
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: DeterminateSystems/nix-installer-action@v14
+
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
+
+      - uses: subosito/flutter-action@v2
+        with:
+          flutter-version: "3.41.6"
+          channel: stable
+          cache: true
+
+      - name: Enter Nix shell and run integration tests
+        run: |
+          nix develop --command bash -c "
+            flutter pub get &&
+            dart run build_runner build --delete-conflicting-outputs &&
+            stalwart-dev/test.sh
+          "
+
+  build-linux:
+    name: Build Linux desktop
+    runs-on: ubuntu-latest
+    needs: analyze-and-test
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install GTK3 and build tools
+        run: |
+          sudo apt-get update -q
+          sudo apt-get install -y --no-install-recommends \
+            libgtk-3-dev pkg-config cmake ninja-build clang
+
+      - uses: subosito/flutter-action@v2
+        with:
+          flutter-version: "3.41.6"
+          channel: stable
+          cache: true
+
+      - name: Install dependencies
+        run: flutter pub get
+
+      - name: Generate Drift code
+        run: dart run build_runner build --delete-conflicting-outputs
+
+      - name: Build Linux release
+        run: flutter build linux --release