Refactor chat scroll coordination

[YeungKC]

Summary

• Move chat scroll ownership out of MessageBloc into a dedicated coordinator that handles restore, loaded-window jumps, visible date tracking, and edge preloading.
• Route quote, pinned message, shared media, selected conversation, and latest-message jumps through the same chat jump helpers.
• Add paging guards for duplicate load-more requests and update the clamping viewport behavior around center changes.
• Keep macOS Profile builds on arm64 because ogg_opus_player includes an arm64-only macOS static library.

Validation

• Ran targeted Flutter analysis for the changed Dart files.
• Ran targeted chat scroll, chat jump, paging, and clamping viewport tests.
• Built the macOS Profile app successfully.
• Ran diff whitespace checks.