Add dynamic sidebar text color with brand color contrast

- Add sidebar text color picker to Branding Settings page
- Implement auto-calculated complementary text colors based on brand color luminance
- Dark themes get light tinted text, light themes get dark tinted text
- Add navigation preview showing text on gradient background
- Support 10 new lighter color palettes (Soft Mint, Lavender, Peach, etc.)
- Add CSS utility classes for brand-text with opacity support
- Update sidebar and navigation components to use dynamic text colors
- Add sidebar_text_color field to Tenant model with migration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/tailwind.config.js

@@ -23,6 +23,8 @@ export default {
                     800: 'var(--color-brand-800, #1e40af)',
                     900: 'var(--color-brand-900, #1e3a8a)',
                 },
+                // Contrast text color for brand backgrounds (supports opacity modifiers like /70)
+                'brand-text': 'rgb(var(--color-brand-text-rgb, 255 255 255) / <alpha-value>)',
             },
         },
     },