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.

app/chat/_layout.tsx

@@ -36,6 +36,8 @@ export default function ChatLayout() {
                                 <View style={{ flexDirection: 'row', gap: 8 }}>
                                     <TouchableOpacity
                                         onPress={() => router.push('/chat/history')}
+                                        accessibilityRole="button"
+                                        accessibilityLabel="Chat history"
                                         style={[styles.headerBtn, { backgroundColor: colors.cardBackground }]}
                                         activeOpacity={0.7}
                                     >
@@ -43,6 +45,8 @@ export default function ChatLayout() {
                                     </TouchableOpacity>
                                     <TouchableOpacity
                                         onPress={clearChat}
+                                        accessibilityRole="button"
+                                        accessibilityLabel="New chat"
                                         style={[styles.headerBtn, { backgroundColor: colors.cardBackground }]}
                                         activeOpacity={0.7}
                                     >
@@ -53,6 +57,8 @@ export default function ChatLayout() {
                             headerRight: () => (
                                 <TouchableOpacity
                                     onPress={() => router.push('/settings')}
+                                    accessibilityRole="button"
+                                    accessibilityLabel="Settings"
                                     style={[styles.headerBtn, { backgroundColor: colors.cardBackground }]}
                                     activeOpacity={0.7}
                                 >