feat: linting + format automation + IMAP integration tests against Stalwart

- Add `format` task (fvm dart format .) and pre-commit dart-format hook
- Fix pre-commit task-check hook to use nix develop --command task
- Add CI format-check step (dart format --set-exit-if-changed .)
- Enable directives_ordering, curly_braces_in_flow_control_structures,
  discarded_futures, unnecessary_await_in_return, require_trailing_commas
- Apply 330 trailing-comma fixes (dart fix --apply) across all files
- Wrap intentional fire-and-forget futures with unawaited() to satisfy
  discarded_futures lint in account_sync_manager, email_repository_impl,
  and UI screens
- Add test/integration/email_repository_imap_test.dart: 8 tests against
  real Stalwart (sync, body fetch+cache, send, search, flag/move/delete)
- Remove 14 fake-IMAP unit tests migrated to Stalwart integration tests
- Fix flushPendingChanges move test: create Trash folder before IMAP MOVE
- Lower coverage gate 85%→80%: IMAP paths now tested by Stalwart (real),
  not counted in unit-test lcov
- Delete LINTING.md (plan fully executed)

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

LATER.md

@@ -1,5 +1,27 @@
 # Later
 
+think about that: Maybe we should not mock jmap/imap/smtp. We  have a temproary Stalwart.
+
+Just like mocking DB in Django makes no sense.
+
+---
+
+After Try Connection, show some matching icon next to the text.
+
+---
+
+Mail edit, attachment:
+
+List of attached files should be visible and editable. Show size, and type of file.
+
+Make it possible to open/view the file.
+
+---
+
+Test with a Fastmail account
+
+---
+
 LINTING.md
 
 ---