feat: improve TUI layout for small screen sizes

[linw1995]

Summary

This PR improves the TUI experience for small terminal sizes by making the UI more responsive and space-efficient.

Changes

Compact Mode for Input Block (chat.rs)

• Added a compact mode that activates when terminal width < 100 characters
• In compact mode, moves context usage indicator and model indicator from bottom to top of the input block
• This balances the visual layout and prevents bottom indicators from overwhelming small screens
• Normal mode (wider screens) keeps the original layout with indicators at the bottom

Adaptive Shortcut Hints Panel (shortcut_hints.rs)

• Made the shortcut hints panel width adaptive to fit available screen space
• Previously fixed at 120+ characters, now shrinks to fit with minimal margins (4 chars reserved)
• Ensures the popup remains usable on smaller screens without horizontal overflow

Related Issue

Fixes #125 - The TUI now adapts layouts to fit available space and ensures essential controls remain accessible on small screen sizes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.67%. Comparing base (a46f02c) to head (6019f8d).
⚠️ Report is 13 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	0.00%	17 Missing ⚠️
crates/coco-tui/src/components/shortcut_hints.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #157      +/-   ##
==========================================
- Coverage   50.22%   47.67%   -2.56%     
==========================================
  Files          93      100       +7     
  Lines       25033    27266    +2233     
==========================================
+ Hits        12574    12999     +425     
- Misses      12459    14267    +1808     

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