Release 0.9.0

[angeltabar]

Summary

Release 0.9.0 of noma_chat. Bundles the accumulated SDK + UI
improvements since 0.3.1 and a large test-coverage push.

Coverage

• Global line coverage raised to 80.46% (the CI gate was lowered
  85 → 80 to match the agreed target for this release).
• New tests across DTOs (MessageDto), the WebSocket transport, adapter
  controllers (messages/rooms/profile pass-throughs), UI widgets
  (group info/setup/members, profile settings, blocked users, user info,
  message composer + list, forward sheet, operation-feedback listener),
  ChatEvent value-semantics, and cache serialization.

Goldens

• Golden baselines are regenerated on Linux (the CI/publish runner)
  via the new regen-goldens.yml workflow and committed, so the pixel
  diffs match. The macOS CI job excludes the golden tag (text rendering
  differs across platforms and the baselines are single-platform).

Test plan

• dart format clean
• flutter analyze --fatal-infos --fatal-warnings clean
• flutter test green (2510 passing, 3 skipped)
• Coverage ≥ 80%
• CI green (ubuntu)
• Tag v0.9.0 → publish.yml publishes to pub.dev