fix(mobile): label the reasoning toggle and give it a full touch target

The toggle now tells a screen reader what it is, "Reasoning: Thought
for 3s", as desktop's prefix does, and reaches a 44 pt target. The
shared English copy stays private to the module that formats it.

mobile/src/session/MobileNativeChatMessage.test.ts

@@ -307,6 +307,9 @@ describe('MobileNativeChatMessage', () => {
       const tree = render(reasoning())
       expect(textIn(tree.root)).toContain('Thought for 3s')
       expect(toggleOf(tree).props.accessibilityState).toEqual({ expanded: false })
+      // Said with what it is, as desktop's screen-reader prefix does, on a 32 + 2 × 6 pt target.
+      expect(toggleOf(tree).props.accessibilityLabel).toBe('Reasoning: Thought for 3s')
+      expect(toggleOf(tree).props.hitSlop).toBe(6)
       expect(markdownIn(tree)).toHaveLength(0)
     })
 
@@ -328,9 +331,10 @@ describe('MobileNativeChatMessage', () => {
 
     it('says only what the host saw', () => {
       expect(textIn(render(reasoning({ state: 'running' })).root)).toContain('Thought')
-      expect(
-        textIn(render(reasoning({ state: undefined, completedAt: undefined })).root)
-      ).toContain('Reasoning')
+      const unknown = render(reasoning({ state: undefined, completedAt: undefined }))
+      expect(textIn(unknown.root)).toContain('Reasoning')
+      // No "Reasoning: Reasoning".
+      expect(toggleOf(unknown).props.accessibilityLabel).toBe('Reasoning')
       expect(textIn(render(reasoning({ completedAt: 1_300 })).root)).toContain('Thought for 1s')
     })
   })

mobile/src/session/MobileNativeChatReasoningRow.tsx

@@ -24,6 +24,8 @@ export function MobileNativeChatReasoningRow({
   onOpenFile?: (relativePath: string) => void
 }): React.JSX.Element {
   const [expanded, setExpanded] = useState(false)
+  const headline = nativeChatReasoningHeadlineText(nativeChatReasoningHeadline(message))
+  const label = nativeChatReasoningHeadlineText({ kind: 'reasoning' })
   return (
     <View>
       <Pressable
@@ -32,9 +34,11 @@ export function MobileNativeChatReasoningRow({
         hitSlop={6}
         accessibilityRole="button"
         accessibilityState={{ expanded }}
+        // Desktop's screen-reader prefix: the headline alone does not say what was thought.
+        accessibilityLabel={headline === label ? label : `${label}: ${headline}`}
       >
         <Text style={styles.reasoningHeadline} numberOfLines={1}>
-          {nativeChatReasoningHeadlineText(nativeChatReasoningHeadline(message))}
+          {headline}
         </Text>
         <View style={expanded ? styles.reasoningCaretOpen : undefined}>
           <ChevronRight size={14} color={colors.textMuted} strokeWidth={2} />

mobile/src/session/mobile-native-chat-message-styles.ts

@@ -36,7 +36,8 @@ export const styles = StyleSheet.create({
     flexDirection: 'row',
     alignItems: 'center',
     gap: spacing.xs,
-    paddingVertical: 3
+    // With the toggle's 6 pt hitSlop above and below, a 44 pt touch target.
+    minHeight: 32
   },
   reasoningPressed: {
     opacity: 0.6

src/shared/native-chat-reasoning-row.ts

@@ -44,7 +44,7 @@ export function nativeChatReasoningHeadline(
 }
 
 /** English copy for clients without a translation catalog; desktop translates the same three. */
-export const NATIVE_CHAT_REASONING_COPY = {
+const NATIVE_CHAT_REASONING_COPY = {
   reasoning: 'Reasoning',
   thought: 'Thought',
   thoughtFor: (duration: string) => `Thought for ${duration}`