docs: add comprehensive color system documentation

COLOR_SYSTEM.md

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+# Boris Color System
+
+All colors now use Tailwind CSS color palette for consistency and maintainability.
+
+## Semantic Color Aliases (Tailwind Config)
+
+```javascript
+'app-bg': '#18181b',           // zinc-900 - Main backgrounds
+'app-bg-elevated': '#27272a',  // zinc-800 - Elevated surfaces (cards, modals)
+'app-bg-subtle': '#1e1e1e',    // Custom ~zinc-850 - Subtle backgrounds
+'app-border': '#3f3f46',       // zinc-700 - Primary borders
+'app-border-subtle': '#52525b', // zinc-600 - Subtle borders
+'app-text': '#e4e4e7',         // zinc-200 - Primary text
+'app-text-secondary': '#a1a1aa', // zinc-400 - Secondary text
+'app-text-muted': '#71717a',   // zinc-500 - Muted text
+'primary': '#6366f1',          // indigo-500 - Primary accent
+'primary-hover': '#4f46e5',    // indigo-600 - Primary hover state
+'highlight-mine': '#fde047',   // yellow-300 - User highlights
+'highlight-friends': '#f97316', // orange-500 - Friends highlights
+'highlight-nostrverse': '#9333ea', // purple-600 - Nostrverse highlights
+```
+
+## Highlight Colors (User-Settable)
+
+Default colors in the color picker:
+- **Yellow** (default): `#fde047` - yellow-300
+- **Orange**: `#f97316` - orange-500
+- **Pink**: `#ec4899` - pink-500
+- **Green**: `#22c55e` - green-500
+- **Blue**: `#3b82f6` - blue-500
+- **Purple**: `#9333ea` - purple-600
+
+## Common Color Mappings
+
+| Old Hex   | Tailwind Color | Usage |
+|-----------|----------------|-------|
+| `#18181b` | zinc-900       | Main app background |
+| `#1a1a1a` | zinc-900       | Component backgrounds |
+| `#1e1e1e` | ~zinc-850      | Code blocks, subtle surfaces |
+| `#252525` | zinc-800       | Hover states |
+| `#27272a` | zinc-800       | Elevated surfaces |
+| `#2a2a2a` | zinc-800       | Buttons, inputs |
+| `#333`    | zinc-700       | Primary borders |
+| `#3f3f46` | zinc-700       | Component borders |
+| `#444`    | zinc-600       | Subtle borders |
+| `#52525b` | zinc-600       | Input borders |
+| `#555`    | zinc-500       | Hover borders |
+| `#666`    | zinc-500       | Muted text |
+| `#71717a` | zinc-500       | Secondary text |
+| `#888`    | zinc-400       | Secondary text |
+| `#999`    | zinc-400       | Muted labels |
+| `#a1a1aa` | zinc-400       | Placeholder text |
+| `#aaa`    | zinc-300       | Light text |
+| `#ccc`    | zinc-300       | Primary text on dark |
+| `#ddd`    | zinc-200       | Bright text |
+| `#e4e4e7` | zinc-200       | Primary text |
+| `#646cff` | indigo-500     | Primary accent |
+| `#535bf2` | indigo-600     | Primary hover |
+| `#fde047` | yellow-300     | Default highlight (brighter) |
+| `#f97316` | orange-500     | Friends highlights |
+| `#9333ea` | purple-600     | Nostrverse highlights |
+
+## Usage Guidelines
+
+### In CSS
+Use Tailwind utilities whenever possible:
+```css
+.example {
+  background: rgb(24 24 27); /* zinc-900 */
+  border: 1px solid rgb(63 63 70); /* zinc-700 */
+  color: rgb(228 228 231); /* zinc-200 */
+}
+```
+
+### In TSX
+Use Tailwind classes:
+```tsx
+<div className="bg-zinc-900 border border-zinc-700 text-zinc-200">
+```
+
+Or semantic aliases:
+```tsx
+<div className="bg-app-bg border border-app-border text-app-text">
+```
+
+### CSS Variables (User-Settable)
+For colors that users can customize:
+```css
+background: var(--highlight-color-mine, #fde047);
+```
+
+## Notes
+
+- All hex colors are now Tailwind palette colors
+- CSS variables remain for user-customizable colors
+- Semantic aliases provide easier maintenance
+- RGB format in CSS allows for opacity control
+