fix(mobile): prevent message text overlap

[techieasif]

Summary

Mobile text messages could leave stale paint over the following message or thread summary after #3370 disabled clipping on every message Material to support gallery overflow.

This change restores Clip.antiAlias for ordinary channel and thread messages while retaining Clip.none only when MessageContent detects an actual trailing multi-image gallery. That keeps gallery cards flush with the trailing gutter without exposing text-only rows to recycled-list paint bleed. A widget regression assertion verifies normal text rows are clipped; the existing gallery assertion continues to verify that galleries are not.

Manual verification: rendered the real mobile channel widget tree at the reporter's 475 px width with two thread summaries and confirmed each message and summary occupies its own row. No follow-up work is currently deferred.

Related issue

Fixes #3491.

Duplicate search: no competing open issue or PR found; #3491 is the closest and originating report.

Testing

• just ci — passed (full Rust, desktop, Tauri, web, and mobile gate; 883 mobile tests passed, 1 skipped)
• flutter test test/features/channels/channel_detail_page_test.dart — 52 passed
• flutter test test/features/channels/message_content_test.dart — 46 passed
• deterministic 475 px mobile widget render used for the after image below

Before	After
Message text and reply summaries overlap.	Message text and reply summaries remain in separate rows.