feat(chat_app): redesign shell and improve model discovery UX

[leehack]

Summary

• Redesign the chat app shell and split provider responsibilities into focused services, including tool declaration handling and runtime diagnostics.
• Expand model management with curated defaults, custom Hugging Face GGUF add flow, popular-model discovery, and bulk management actions.
• Update web bridge asset pinning to v0.1.4 so the app uses the latest bridge runtime behavior.

Notes

• Branch includes multiple chat app commits (shell redesign, provider refactor, declaration-based tool flow, discovery UX updates).
• Local unrelated working-tree edits are intentionally not part of this PR.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.91%. Comparing base (2e9d0d5) to head (ace4331).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/backends/llama_cpp/llama_cpp_service.dart	76.92%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   76.37%   76.91%   +0.53%     
==========================================
  Files          65       65              
  Lines        7915     7930      +15     
==========================================
+ Hits         6045     6099      +54     
+ Misses       1870     1831      -39     

Flag	Coverage Δ
unittests	76.91% <83.33%> (+0.53%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.