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/chat/ModelSelector.tsx

@@ -42,6 +42,8 @@ export function ModelSelector() {
                 style={[styles.headerButton, { backgroundColor: colors.cardBackground }]}
                 onPress={() => setModalVisible(true)}
                 activeOpacity={0.7}
+                accessibilityRole="button"
+                accessibilityLabel={`Model: ${activeModelName}. Tap to change`}
             >
                 <ConnectionBadge isConnected={isConnected} compact />
                 <Text style={[styles.headerText, { color: colors.text }]} numberOfLines={1}>
@@ -70,6 +72,8 @@ export function ModelSelector() {
                                 onPress={handleRefresh}
                                 style={[styles.refreshButton, { backgroundColor: colors.cardBackground }]}
                                 disabled={isRefreshing}
+                                accessibilityRole="button"
+                                accessibilityLabel="Refresh models"
                             >
                                 {isRefreshing ? (
                                     <ActivityIndicator size="small" color={colors.textMuted} />
@@ -108,6 +112,9 @@ export function ModelSelector() {
                                             isSelected && { borderColor: colors.tint }
                                         ]}
                                         onPress={() => handleSelect(item.id)}
+                                        accessibilityRole="button"
+                                        accessibilityLabel={item.id}
+                                        accessibilityState={{ selected: isSelected }}
                                     >
                                         <View style={styles.itemContent}>
                                             <Text style={[styles.itemName, { color: colors.text }]}>