#21393 refactor(home): replace Home chat bar with clean Ask Anything button (#19854)

Description

Resolves #19854.

Cleans up the Home bottom chat bar by introducing a centered, dedicated HomeAskAnythingButton pill while preserving bottom bar height (kHomeChatBarHeight = 62.0), container borders, background styling, and the adjacent HomeRecordButton.

Key Changes

• app/lib/widgets/home_bottom_bar.dart: Created reusable HomeAskAnythingButton component displaying centered localized text (context.l10n.askAnything), removing the voice circle icon and speech bubble icon from the Home bottom bar. Tapping triggers the existing chat sheet callback (openChatSheet). Voice input remains accessible inside the opened chat sheet / composer.
• app/lib/pages/home/page.dart: Replaced inline _buildChatBar button layout with HomeAskAnythingButton.
• app/integration_test/visual_audit/scenarios/capture.dart: Replaced custom _AskOmiBar with shared HomeAskAnythingButton to keep test harness and production UI unified.
• app/test/widgets/home_ask_anything_button_test.dart: Hermetic widget tests verifying:
  • Centered localized text rendering.
  • Absence of microphone or chat icons.
  • Correct tap handling triggering the provided callback.
  • Proper theme-responsive container styling in light and dark modes.
• app/changelog/unreleased/20261013-home-bottom-bar-ask-anything.json: Added unreleased changelog fragment.

Testing

• Hermetic Flutter widget tests: flutter test test/widgets/home_ask_anything_button_test.dart

UI-Evidence: none