fix(ui): add aria-label to STT button and aria-expanded to pinned toggle (#70593)

- Voice input button had title= but no aria-label, so screen readers
  announced it without context. Add aria-label mirroring the title,
  toggling between "Voice input" and "Stop recording".
- Pinned messages toggle lacked aria-expanded, so screen readers could
  not announce the collapsed/expanded state of the section.

Co-authored-by: akinshaywai <akinshaywai@users.noreply.github.com>

ui/src/ui/views/chat.ts

@@ -572,6 +572,7 @@ function renderPinnedSection(
     <div class="agent-chat__pinned">
       <button
         class="agent-chat__pinned-toggle"
+        aria-expanded=${vs.pinnedExpanded}
         @click=${() => {
           vs.pinnedExpanded = !vs.pinnedExpanded;
           requestUpdate();
@@ -1251,6 +1252,7 @@ export function renderChat(props: ChatProps) {
                       }
                     }}
                     title=${vs.sttRecording ? "Stop recording" : "Voice input"}
+                    aria-label=${vs.sttRecording ? "Stop recording" : "Voice input"}
                     ?disabled=${!props.connected}
                   >
                     ${vs.sttRecording ? icons.micOff : icons.mic}