feat: user bubbles now use neutral gray background

- User messages: var(--bg-tertiary) instead of accent color
- Assistant messages: var(--bg-secondary) (unchanged)
- User bubbles: slightly lighter gray (1f1f1f) than assistant (1a1a1a)
- User bubble border: var(--border-light) for subtle distinction
- User label color: var(--text-secondary) instead of white
- Both message types now have consistent text color (var(--text-primary))
- Streaming bubble unchanged (assistant style)
- All 52 tests passing

src/components/ChatWindow.tsx

@@ -300,18 +300,18 @@ export function ChatWindow({ session, config, onAddMessage }: ChatWindowProps) {
                     ? 'var(--radius-lg) var(--radius-lg) 4px var(--radius-lg)'
                     : 'var(--radius-lg) var(--radius-lg) var(--radius-lg) 4px',
                   backgroundColor: msg.role === 'user'
-                    ? 'var(--accent)'
+                    ? 'var(--bg-tertiary)'
                     : 'var(--bg-secondary)',
-                  color: msg.role === 'user' ? 'white' : 'var(--text-primary)',
+                  color: 'var(--text-primary)',
                   boxShadow: 'var(--shadow)',
-                  border: `1px solid ${msg.role === 'user' ? 'var(--accent)' : 'var(--border)'}`
+                  border: `1px solid ${msg.role === 'user' ? 'var(--border-light)' : 'var(--border)'}`
                 }}
               >
                 <div style={{
                   fontSize: '12px',
                   fontWeight: 600,
                   marginBottom: '8px',
-                  color: msg.role === 'user' ? 'rgba(255,255,255,0.8)' : 'var(--accent)'
+                  color: msg.role === 'user' ? 'var(--text-secondary)' : 'var(--accent)'
                 }}>
                   {msg.role === 'user' ? 'You' : 'Assistant'}
                 </div>