Fix search button cascade in light theme

[pitill0]

Summary

Fixes the Search submit button styling in the light theme by resolving the actual CSS specificity conflict.

Root cause

The shared light-theme rule in buttons.css:

html[data-theme="light"] button

had greater specificity than the Search-specific rule in search.css:

.search-form button

As a result, the Search button reverted to the generic light-theme appearance and hover behavior.

Changes

• Moves the Search primary-action colors to buttons.css.
• Uses a selector with sufficient specificity to win consistently in both themes.
• Removes duplicated Search color ownership from search.css.
• Preserves the responsive mobile width rule.
• Adds regression coverage for the shared Search button styling without confusing visual ownership with responsive layout.

Validation

• Search button matches the primary-action palette.
• Hover contrast is correct in light theme.
• Dark theme behavior remains unchanged.
• Mobile layout remains unchanged.
• Full test suite passes: 876 tests.