fix(canvas/mobile): aria-label on MobileChat composer textarea and MobileSpawn name input (WCAG 1.3.1)

MobileChat: composer textarea had no aria-label — added aria-label="Message".
MobileSpawn: name input had no programmatic label — added aria-label="Agent name".

Both inputs had visible text labels above them but no accessible-name association,
violating WCAG 1.3.1 (info/structure relationships).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/mobile/MobileChat.tsx

@@ -725,6 +725,7 @@ export function MobileChat({
             ref={composerRef}
             value={draft}
             onChange={(e) => setDraft(e.target.value)}
+            aria-label="Message"
             onKeyDown={(e) => {
               // Enter sends; Shift+Enter inserts a newline. Skip when the
               // IME is composing — pressing Enter to commit a Chinese/

canvas/src/components/mobile/MobileSpawn.tsx

@@ -302,6 +302,7 @@ export function MobileSpawn({ dark, onClose }: { dark: boolean; onClose: () => v
           <input
             value={name}
             onChange={(e) => setName(e.target.value)}
+            aria-label="Agent name"
             placeholder={tplId
               ? (templates.find((t) => t.id === tplId)?.name ?? "agent-name")
               : "agent-name"}