Improve chat app download reliability and streaming performance

[leehack]

Summary

• improve chat app model download UX with stage-aware progress, transfer-rate feedback, pause/cancel lifecycle handling, and mobile platform config updates
• optimize chat streaming responsiveness by reducing no-op UI updates, coalescing auto-follow scrolling, and using a plain-text render fast path when markdown is not needed
• reduce web pre-download memory usage by verifying remote assets instead of buffering full GGUF payloads, and add settings fallback hardening plus regression/docs/changelog updates

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.26%. Comparing base (c7c0ec0) to head (c542ef9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #62   +/-   ##
=======================================
  Coverage   77.26%   77.26%           
=======================================
  Files          66       66           
  Lines        8046     8046           
=======================================
  Hits         6217     6217           
  Misses       1829     1829           

Flag	Coverage Δ
unittests	77.26% <ø> (ø)

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