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/CodeBlock.tsx

@@ -16,7 +16,11 @@ export function CodeBlock({ language, content, copied, onCopy }: CodeBlockProps)
         <View style={styles.codeBlockWrapper}>
             <View style={styles.codeBlockHeader}>
                 <Text style={styles.codeBlockLanguage}>{language}</Text>
-                <TouchableOpacity onPress={() => onCopy(content)} style={styles.copyButton}>
+                <TouchableOpacity
+                    onPress={() => onCopy(content)}
+                    style={styles.copyButton}
+                    accessibilityRole="button"
+                >
                     <CopyIcon size={14} color="#A0A0A0" />
                     <Text style={styles.copyText}>{copied ? 'Copied!' : 'Copy'}</Text>
                 </TouchableOpacity>