Fix editing message background fill not stretching with more lines

[ghostiee-11]

Fixes #603

Changes

When a user edits a chat message, the Paper background now stretches to full width instead of staying at fit-content.

Root cause

The isResponsive check in ChatMessage.jsx reads sizing_mode from the child model, but this check is not reactive when _object_panel is swapped to the edit area. The Paper stays at fit-content, causing the edit textarea to overflow its background.

Fix

Python (message.py):

• Set sizing_mode='stretch_width' on the ChatAreaInput edit area
• Add placeholder="Edit message..." for UX clarity

JSX (ChatMessage.jsx):

• Add a MutationObserver on the Paper element that detects when the .edit-area CSS class appears in the DOM
• When editing, set isEditing=true which forces Paper to width: 100%
• When edit is cancelled/submitted, observer detects removal and reverts to fit-content
• Add position: relative; z-index: 1 on the edit/copy icon Stack to prevent overlap from next message

Tests (test_message_edit.py):

• Add test_edit_area_has_stretch_width - verifies sizing_mode='stretch_width'
• Add test_edit_area_has_edit_css_class - verifies edit-area CSS class presence (required for MutationObserver detection)

Screenshot

After

Edit area stretches to full width with clean underline styling. All text contained within the Paper background.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.66%. Comparing base (647425f) to head (428e322).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #604      +/-   ##
==========================================
- Coverage   82.03%   81.66%   -0.38%     
==========================================
  Files          31       31              
  Lines        3229     3229              
==========================================
- Hits         2649     2637      -12     
- Misses        580      592      +12     

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[ahuang11]

I can't get it working after pixi run compile-dev and clearing cache and restarting browser. Were you able to get it to work?

[philippjfr]

Definitely still overflows. Will investigate and push to this PR to fix.