Fix LVGL object deletion crash and thread safety issues in SetChatMessage (#1708)

* Initial plan

* Fix LVGL object deletion issue in SetChatMessage

- Refresh child_count after deleting first_child to avoid using stale count
- Add lv_obj_is_valid() checks before accessing deleted objects
- Prevent duplicate deletion by refreshing child_count before system message deletion
- Ensures last_child validation before scrolling to it

Co-authored-by: 78 <4488133+78@users.noreply.github.com>

* Address code review feedback on LVGL object deletion fix

- Move last_child retrieval after deletion to avoid stale pointer
- Add child_count > 0 check before scrolling to prevent edge case issues
- Add lv_obj_is_valid() check for last_bubble for consistency

Co-authored-by: 78 <4488133+78@users.noreply.github.com>

* Final review: LVGL object deletion fix complete

Co-authored-by: 78 <4488133+78@users.noreply.github.com>

* Remove std::thread wrappers in display callbacks to fix thread safety

Remove detached threads that were calling SetChatMessage from download/upgrade
progress callbacks. The DisplayLockGuard mutex already provides thread safety,
so we can call display methods directly from the callback thread without
spawning additional threads, which were causing race conditions.

Co-authored-by: 78 <4488133+78@users.noreply.github.com>

* Use Application::Schedule() for display updates in callbacks

Instead of calling SetChatMessage directly from download/upgrade progress
callbacks, queue the updates using Application::Schedule(). This prevents
blocking the network receive task and avoids potential UART FIFO overflow
issues with 4G modems. The scheduled callbacks execute in the main task
thread, maintaining proper thread safety.

Co-authored-by: 78 <4488133+78@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: 78 <4488133+78@users.noreply.github.com>

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