Initial Flutter/Dart port of SharedInbox

IMAP/SMTP email client with offline-first architecture:
sync engine writes to Drift (SQLite), UI reads reactively
from the local DB. enough_mail vendored under packages/.

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

AGENTS.md

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+# SharedInbox Flutter — Agent Guide
+
+## Code conventions
+
+- No `if` chains where a pattern/match or data-driven approach works.
+- Fail loudly — `throw StateError(...)` beats silent fallbacks.
+- New source files go under `lib/core/` (interfaces/models), `lib/data/` (implementations), or `lib/ui/` (screens/widgets).
+
+## Drift (DB)
+
+- Schema in `lib/data/db/database.dart`.
+- After any schema change run: `dart run build_runner build --delete-conflicting-outputs`
+- Generated `database.g.dart` is committed — do not hand-edit it.
+
+## enough_mail (vendored)
+
+- Located at `packages/enough_mail/` — edit freely.
+- IMAP client helpers are in `lib/data/imap/imap_client_factory.dart`.
+
+## Running
+
+```bash
+# Code generation (must run after schema changes)
+dart run build_runner build --delete-conflicting-outputs
+
+# Desktop
+flutter run -d linux
+
+# Tests
+flutter test
+```
+
+## Adding a screen
+
+1. Create `lib/ui/screens/my_screen.dart`.
+2. Add a `GoRoute` in `lib/ui/router.dart`.
+3. No separate ViewModel file needed — use `ConsumerWidget` / `ConsumerStatefulWidget` directly with Riverpod providers.