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}
                                 >

app/chat/history.tsx

@@ -20,7 +20,12 @@ export default function HistoryScreen() {
     return (
         <View style={[styles.container, { backgroundColor: colors.backgroundColor }]}>
             <View style={[styles.header, { borderBottomColor: colors.border, backgroundColor: colors.backgroundColor }]}>
-                <TouchableOpacity onPress={() => router.back()} style={styles.backButton}>
+                <TouchableOpacity
+                    onPress={() => router.back()}
+                    style={styles.backButton}
+                    accessibilityRole="button"
+                    accessibilityLabel="Back"
+                >
                     <ArrowLeft size={24} color={colors.text} />
                 </TouchableOpacity>
                 <Text style={[styles.title, { color: colors.text }]}>Chat History</Text>
@@ -43,6 +48,8 @@ export default function HistoryScreen() {
                                 { backgroundColor: colors.cardBackground, borderColor: colors.border }
                             ]}
                             onPress={() => handleLoadChat(item.id)}
+                            accessibilityRole="button"
+                            accessibilityLabel={`Open chat: ${item.title}`}
                         >
                             <View style={styles.chatInfo}>
                                 <Text style={[styles.chatTitle, { color: colors.text }]} numberOfLines={1}>
@@ -55,6 +62,8 @@ export default function HistoryScreen() {
                             <TouchableOpacity
                                 style={styles.deleteButton}
                                 onPress={() => deleteChatSession(item.id)}
+                                accessibilityRole="button"
+                                accessibilityLabel={`Delete chat: ${item.title}`}
                             >
                                 <Trash2 size={20} color={colors.errorText} />
                             </TouchableOpacity>