feat(a11y): label icon-only buttons and inputs for screen readers

Header, message action, send/stop, history, model selector and dark-mode controls had no accessible name (icon-only buttons). Adds roles/labels and selected/disabled state; no visual change.

components/settings/AppearanceCard.tsx

@@ -29,6 +29,7 @@ export function AppearanceCard() {
                 <Switch
                     value={isDarkMode}
                     onValueChange={toggleTheme}
+                    accessibilityLabel="Dark Mode"
                     trackColor={{ false: '#E2E8F0', true: colors.tint }}
                     thumbColor={'#FFFFFF'}
                 />