feat: Story 9 - Remove scroll bars for cleaner UI

Implemented Story 9: Hidden Scroll Bars
- Added CSS to hide scroll bars globally while maintaining functionality
- Used scrollbar-width: none for Firefox
- Used ::-webkit-scrollbar { display: none; } for Chrome/Safari/Edge
- Set overflow-x: hidden to prevent unwanted horizontal scrolling
- Maintained full scroll functionality (mouse wheel, trackpad, keyboard)

Updated Specs
- Added Scroll Bar Styling section to UI_UX.md
- Documented cross-browser compatibility requirements
- Specified areas affected and implementation approach

Bug Fixes
- Fixed biome linting issues in App.tsx (import organization, button type)
- Applied biome formatter for consistent code style

.living_spec/specs/functional/UI_UX.md

@@ -53,3 +53,32 @@ Tool outputs should be rendered in a collapsible component that is **closed by d
 *   Or implement custom collapsible component with proper ARIA attributes
 *   Tool outputs should be visually distinct (border, background color, or badge)
 *   Multiple tool calls in sequence should each be independently collapsible
+
+## Scroll Bar Styling
+
+### Problem
+Visible scroll bars create visual clutter and make the interface feel less polished. Standard browser scroll bars can be distracting and break the clean aesthetic of the dark theme.
+
+### Solution: Hidden Scroll Bars with Maintained Functionality
+Scroll bars should be hidden while maintaining full scroll functionality.
+
+### Requirements
+
+1. **Visual:** Scroll bars should not be visible to the user
+2. **Functionality:** Scrolling must still work perfectly:
+   - Mouse wheel scrolling
+   - Trackpad scrolling
+   - Keyboard navigation (arrow keys, page up/down)
+   - Auto-scroll to bottom for new messages
+3. **Cross-browser:** Solution must work on Chrome, Firefox, and Safari
+4. **Areas affected:**
+   - Main chat message area (vertical scroll)
+   - Tool output content (both vertical and horizontal)
+   - Any other scrollable containers
+
+### Implementation Notes
+*   Use CSS `scrollbar-width: none` for Firefox
+*   Use `::-webkit-scrollbar { display: none; }` for Chrome/Safari/Edge
+*   Maintain `overflow: auto` or `overflow-y: scroll` to preserve scroll functionality
+*   Ensure `overflow-x: hidden` where horizontal scroll is not needed
+*   Test with very long messages and large tool outputs to ensure no layout breaking