Improve desktop chat rendering and navigation performance

[YeungKC]

Summary

• Replace the remaining Bloc/Cubit chat state paths with lighter notifiers and local state storage.
• Refactor desktop chat navigation so route mode, side panels, and conversation focus have clearer ownership.
• Rework message window loading, jump handling, and scroll coordination for quote, pinned-message, latest, and repeated conversation selection.
• Split oversized chat/message rendering files into smaller helpers for content, actions, rows, date overlays, and shared media lists.
• Improve media image handling with a dedicated pipeline for cached images, avatar refresh, GIF frame normalization, and safer streamed downloads.
• Stabilize message blink highlighting so it paints over bubble content without remounting message widgets or flickering mention names.

Validation

• Ran the affected Flutter widget/unit tests locally.
• Ran Dart analysis locally.
• Fixed formatting and confirmed the PR checks are passing.