fix(web): unify primary button palette

[pitill0]

Summary

This PR restores a single shared visual style for primary Web actions across dark and light themes.

Changes

• Restores the same --action-fill value in both themes.
• Removes light-theme-specific primary button overrides from:
  • Search
  • Login
  • Initial setup
  • Play station
  • Admin forms
• Keeps the existing shared hover behavior.
• Preserves the valid light-theme overrides for:
  • diagnostic surfaces;
  • destructive actions;
  • Admin rows and form surfaces.
• Updates the regression tests to ensure primary actions continue using the shared palette without theme-specific duplication.

Root cause

The previous light-theme consistency change introduced a separate primary action color and repeated component-specific overrides.

That made Search, Login, Play and Admin buttons diverge from the shared primary button pattern, even though they represent the same type of action.

Impact

Primary actions now use the same color, text contrast and hover behavior in both themes and at all viewport sizes, including mobile.

No HTML, JavaScript or responsive layout behavior has changed.

Validation

• Search matches the other primary actions.
• Login and setup match Search.
• Play station uses the shared primary style.
• Admin form buttons use the shared primary style.
• Hover behavior remains consistent.
• Light-theme destructive and diagnostic overrides remain intact.
• Mobile layout remains unchanged.
• python -m pytest tests/test_web_light_theme_css.py
• Full CI passes.