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>

Taskfile.yml

@@ -2,7 +2,7 @@ version: "3"
 
 tasks:
   default:
-    desc: Run all checks (analyze + test, in parallel)
+    desc: Run all checks (analyze + unit tests, in parallel)
     deps: [check]
 
   _nix-check:
@@ -13,7 +13,7 @@ tasks:
         msg: "Not in nix dev shell. Run: nix develop"
 
   codegen:
-    desc: Run build_runner to generate Drift and Riverpod code
+    desc: Generate Drift DB code (run after any schema change)
     deps: [_nix-check]
     cmds:
       - |
@@ -23,7 +23,7 @@ tasks:
         echo "codegen: $((END - START))s"
 
   analyze:
-    desc: Run flutter analyze
+    desc: Static analysis with flutter analyze + analysis_options.yaml rules
     deps: [_nix-check]
     cmds:
       - |
@@ -33,20 +33,30 @@ tasks:
         echo "analyze: $((END - START))s"
 
   analyze-fix:
-    desc: Auto-fix with dart fix --apply
+    desc: Auto-fix lint issues with dart fix --apply
     deps: [_nix-check]
     cmds:
       - dart fix --apply
 
   test:
-    desc: Run unit tests
+    desc: Run pure-Dart unit tests (no device needed)
+    deps: [_nix-check]
+    cmds:
+      - |
+        START=$(date +%s)
+        dart test test/unit/
+        END=$(date +%s)
+        echo "test: $((END - START))s"
+
+  test-flutter:
+    desc: Run Flutter widget tests
     deps: [_nix-check]
     cmds:
       - |
         START=$(date +%s)
         flutter test
         END=$(date +%s)
-        echo "test: $((END - START))s"
+        echo "flutter test: $((END - START))s"
 
   integration:
     desc: Run integration tests (starts and stops Stalwart automatically)
@@ -65,5 +75,5 @@ tasks:
       - flutter run -d linux
 
   check:
-    desc: All checks — analyze + unit tests in parallel
+    desc: All fast checks — analyze + unit tests in parallel
     deps: [analyze, test]