Use Material3 for Assist conversation

[TimoPtr]

Summary

This PR is a simple update of the UI of Assist bottom sheet to look more aligned with the rest of the app.

Checklist

• New or updated tests have been added to cover the changes following the testing guidelines.
• The code follows the project's code style and best_practices.
• The changes have been thoroughly tested, and edge cases have been considered.
• Changes are backward compatible whenever feasible. Any breaking changes are documented in the changelog for users and/or in the code for developers depending on the relevance.
• I have read the Open Home Foundation AI Policy.

Select exactly one option that describes AI usage in this contribution:

• I have not used AI for this contribution.
• AI assistance was used for this contribution.
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Screenshots

[Copilot]

Pull request overview

This PR modernizes the Assist bottom sheet conversation UI to align with the app’s Material3/HA design system components, replacing the legacy Material2-based implementation and adding screenshot coverage for key states.

Changes:

• Replaced the legacy AssistSheetView (Material2 ModalBottomSheetLayout) with a new AssistSheet built on HAModalBottomSheet and HA theme tokens.
• Added screenshot tests covering multiple Assist sheet states (voice/text, multi-server pipelines, error, blocked, long conversation).
• Updated Assist activity theming and changelog entries to reflect the UI modernization.

Reviewed changes

Copilot reviewed 6 out of 48 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
app/src/main/kotlin/io/homeassistant/companion/android/assist/ui/AssistSheet.kt	New Material3/HA-themed Assist bottom sheet implementation (header, conversation list, input controls).
app/src/main/kotlin/io/homeassistant/companion/android/assist/ui/AssistSheetView.kt	Removed legacy Material2-based Assist sheet UI.
app/src/main/kotlin/io/homeassistant/companion/android/assist/AssistActivity.kt	Switched to HATheme and updated to use AssistSheet.
app/src/main/kotlin/io/homeassistant/companion/android/assist/AssistViewModel.kt	Adjusted exposed conversation list to hide input placeholder during active voice recording.
app/src/screenshotTest/kotlin/io/homeassistant/companion/android/assist/ui/AssistSheetScreenshotTest.kt	Added screenshot tests for the updated Assist sheet UI across multiple states.
app/src/main/res/xml/changelog_master.xml	Added changelog entries noting Assist conversation screen modernization (mobile + automotive).

[jpelgrom]

The sheet has the app title when opened from outside the app, can you add a screenshot test for that?