feat(flutter): add API client and Reminder model

[kauereinbold]

Summary

• lib/api/reminder.dart: Reminder model (id, title, description, limitDate, isDone), fromJson/toJson with date-only UTC limitDate (yyyy-MM-ddT00:00:00Z, accepts Z and +00:00 inputs), copyWith, value equality.
• lib/api/api_client.dart: RemindersApi over the http package (new dependency http ^1.2.0, the standard Flutter HTTP client, testable with MockClient): fetchAll, fetchOne, create, update, delete, toggleDone. ApiException with status code, message and per-field errors, mapping the .NET problem-details errors map and the Go/C++ {message} shape; isNotFound, isValidation, isNetwork.
• Base URL from --dart-define=API_BASE_URL, default http://10.0.2.2:9999 (Android emulator to host nginx).
• Tests: 7 model cases and 15 client cases (request method, URL, headers, body, error mapping, network failure, trailing-slash base URL). README structure section updated.

Parent: #325. Next: #338 list screen consumes this client.

Closes #337

Test plan

• flutter analyze clean, flutter test 23/23, dart format applied
• Live round trip against the local stack (--dart-define=API_BASE_URL=http://localhost:9999, temporary test not committed): create, list, fetch, toggle, delete, 404 after delete, across the load-balanced .NET/Go/C++ APIs
• CI Build and Test Flutter App green

[github-actions]

Reviewed diff. No prior review comments found.

[minor]

• src/app/flutter/reminders_app/pubspec.yaml:14: new http dep, no ADR. CLAUDE.md says new dependency needs ADR. Add one or note why skipped (standard Flutter HTTP pkg, testable via MockClient).
• src/app/flutter/reminders_app/lib/api/api_client.dart:123: success path jsonDecode uncaught. Malformed 200 body throws raw FormatException, not ApiException. Wrap in try/catch like _errorFrom does.

Contract check: verified id/title/description/limitDate/isDone shape and error shape (.NET problem-details errors map, Go/C++ {message}) match across all three backend APIs, incl. C++ to_char(... 'YYYY-MM-DD"T"HH24:MI:SS"Z"') producing the same Z-suffixed format the Dart parser expects. No cross-API contract bugs found.

No em/en dashes, PR title conventional, no AI attribution, no schema changes (no migrations needed), tests cover model + client thoroughly.

Ready for human review.

[kauereinbold]

Addressed in 547378f: success-path jsonDecode now maps malformed bodies to ApiException (test added), ADR-0009 records the http package choice.

[github-actions]

Re-reviewed full diff (both commits). Prior findings addressed: success-path jsonDecode now wrapped in try/catch (test added), ADR-0009 covers the http dep choice.

[minor]

• src/app/flutter/reminders_app/pubspec.lock: lock bumps ~10 unrelated transitive deps (leak_tracker, matcher, vector_math, etc.) and raises dart: ">=3.7.0" to ">=3.9.0-0". CI pins Flutter 3.29.0 (.github/workflows/flutter-pull-request.yml:27), which ships Dart 3.7. Lock looks generated with a newer local Flutter/Dart than CI uses. Regenerate the lock with Flutter 3.29.0 so committed lock matches what CI resolves and what was actually tested.

No blockers or majors. Contract shapes, error mapping, tests, ADR, no dashes, conventional title, no AI attribution all check out.

Ready for human review.