Enforce Dart formatting in CI

[vk0eppel]

What to build

Unformatted Dart should fail the PR, matching the existing cargo fmt --check gate for Rust. There is currently no Dart formatting check in CI, so dart format drift can land unnoticed.

Acceptance criteria

• The Flutter CI job runs dart format --output=none --set-exit-if-changed . (or equivalent) and fails on unformatted code.
• The existing tree is formatted so the new check passes on main (one-time reformat commit if needed).
• CONVENTION.md's checks list mentions the Dart format command alongside the Rust one.

Blocked by

None — can start immediately.

[vk0eppel]

Done in #202 (merged to main as cb727dc). GitHub only auto-closed the first issue in the PR's comma-separated Closes list, hence the manual close.