a11y(coach): announce streaming assistant text to screen readers

The streaming assistant bubble updated in place as the SSE tokens
landed but had no aria-live region — sighted users saw live text
while screen-reader users heard silence. Wrap the in-flight bubble
in role=log + aria-live=polite + aria-relevant=additions text so the
assistant prose announces progressively (matches the daily-briefing
pattern that already uses aria-live for its loading state).

Fixes Design-HIGH-3.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/insights/coach-panel/message-thread.tsx

@@ -115,14 +115,20 @@ export function MessageThread({
         />
       ))}
       {streamingActive && streaming && (
-        <ChatBubble
-          role="assistant"
-          content={streaming.content}
-          metricSource={streaming.metricSource}
-          providerType={streaming.inProgress ? "streaming" : null}
-          inProgress={streaming.inProgress}
-          errorCode={streaming.errorCode}
-        />
+        // role=log + aria-live=polite so screen-reader users hear the
+        // assistant prose announce as tokens land. aria-relevant=text
+        // limits announcements to the streamed content; additions
+        // covers the bubble-mount edge case.
+        <div role="log" aria-live="polite" aria-relevant="additions text">
+          <ChatBubble
+            role="assistant"
+            content={streaming.content}
+            metricSource={streaming.metricSource}
+            providerType={streaming.inProgress ? "streaming" : null}
+            inProgress={streaming.inProgress}
+            errorCode={streaming.errorCode}
+          />
+        </div>
       )}
     </div>
   );