Add dark theme support for JSONEditor widget (#7441)

[AtharvaJaiswal005]

Summary

• Override upstream jsoneditor@10.2.0 hardcoded CSS colors with Panel CSS custom properties (--panel-background-color, --panel-on-background-color, --panel-surface-color, etc.) so the JSONEditor widget adapts to both light and dark themes automatically
• Covers: tree/text backgrounds, field and value text, value type syntax highlighting (string, number, boolean, null, url), menu bar, navigation bar, status bar, treepath breadcrumb, search box, highlight/selection states, context menus, popovers/tooltips, autocomplete dropdown, modal dialogs (sort, transform), form inputs, validation errors, empty placeholders, and "show more" indicators

Changes

• panel/models/jsoneditor.ts - Import and push the new theme-adaptive stylesheet into the shadow DOM
• panel/styles/models/jsoneditor.less - New 269-line LESS file that maps all jsoneditor hardcoded colors to Panel CSS variables with light-theme fallback defaults

How it works

The LESS file uses var(--panel-background-color, #ffffff) style rules that:

• Fall back to the original light-theme defaults when no variables are set
• Automatically pick up dark-theme values (--panel-background-color: #212529, etc.) when Panel runs with a dark template/theme
• No JavaScript changes needed - pure CSS variable inheritance through the shadow DOM

Test plan

• Run panel serve with FastListTemplate(theme="dark") and verify JSONEditor has dark background, white text, themed menus
• Switch to theme="default" and verify light theme still looks correct (no regression)
• Test tree mode, text mode, and form mode
• Right-click context menu should have themed background/text
• Search box, navigation bar, status bar should match theme
• Existing panel/tests/ui/widgets/test_jsoneditor.py tests should pass

Closes #7441

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.05%. Comparing base (9c86ad7) to head (f88e5ea).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8444      +/-   ##
==========================================
- Coverage   86.16%   86.05%   -0.11%     
==========================================
  Files         349      349              
  Lines       55184    55184              
==========================================
- Hits        47548    47489      -59     
- Misses       7636     7695      +59     

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