Add dark mode styles for KB fact detail page form elements

The /kb/<id> fact detail page has form elements (textarea, inputs) and a danger-zone delete box that use light theme colors. In dark mode these appear with white backgrounds and dark borders, making them hard to read.

Components needing dark mode styles in Style.hs darkModeStyles:

• .form-input and .form-textarea (lines 845-858) - need dark bg (#374151), dark border (#4b5563), light text (#f3f4f6)
• .form-group label (line 839) - needs light text color (#d1d5db)
• .danger-zone (lines 884-892) - uses light pink bg (#fef2f2) and pink border (#fecaca), needs dark red theme (e.g., bg #450a0a, border #991b1b)
• .danger-zone h2 - red heading should remain visible in dark mode

Pattern to follow: see existing dark mode selectors like .filter-input, .comment-textarea that have proper dark mode colors.