fix(frontend): disable 'Altro' Send until non-whitespace text (no empty freetext)

Backlog T2 from the reviewer-gate-ux review: the Send button on the reserved
"Other — specify" textarea emitted control:freetext with an empty payload when
clicked on an empty/whitespace field. Guard with disabled={!text.trim()}.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/widgets/ReservedControls.tsx

@@ -32,7 +32,11 @@ export function ReservedControls({
             value={text}
             onChange={(e) => setText(e.target.value)}
           />
-          <button className="text-sm border rounded px-2 py-1 self-start" onClick={() => onControl("freetext", text)}>
+          <button
+            className="text-sm border rounded px-2 py-1 self-start disabled:opacity-50 disabled:cursor-not-allowed"
+            disabled={!text.trim()}
+            onClick={() => onControl("freetext", text)}
+          >
             Send
           </button>
         </div>