fix(web): harmonize light theme colors

[pitill0]

Summary

This PR improves color consistency across the FluxTuner Web light theme.

Changes

• Defines the missing --help-text token for both dark and light themes.
• Uses a consistent light-theme primary action style for:
  • Search
  • Login
  • Initial setup
  • Play station
  • Admin forms
• Updates light-theme destructive actions:
  • Reject
  • Delete user
  • Delete playlist
  • Remove from playlist
• Adapts the search debug panel and its internal cards to light surfaces.
• Updates Admin user forms and rows so they no longer retain dark translucent backgrounds.
• Adds focused regression tests for light-theme tokens and component overrides.

Root cause

Several Web components defined their own dark-oriented colors after the shared button styles.

Some of those components had no light-theme override, while others used a different light primary-action style. This resulted in inconsistent buttons and dark surfaces appearing inside the light theme.

The --help-text variable was also referenced without being defined, causing those declarations to become invalid and allowing inherited colors to vary by context.

Impact

The light theme now presents a more consistent visual hierarchy:

• primary actions use the same palette;
• secondary actions remain neutral;
• destructive actions use appropriate light red surfaces;
• diagnostic and Admin surfaces no longer retain dark-theme backgrounds.

No HTML structure, responsive layout or JavaScript behavior has been changed.

Validation

• Search button is consistent in normal and hover states.
• Login and setup primary actions match Search.
• Play station uses the same light primary-action palette.
• Admin form actions are consistent.
• Reject, Delete and playlist removal actions are readable in light mode.
• Search debug surfaces are correctly themed.
• Desktop layout remains unchanged.
• Mobile layout remains unchanged.
• python -m pytest tests/test_web_light_theme_css.py
• Full CI passes.