feat(chat): stop an in-flight answer — Send becomes Stop, the partial is kept and persisted, the model stream is torn down

frontend/index.html

@@ -125,7 +125,23 @@
         </div>
       </section>
 
-      <form class="composer" id="composer">
+      <!-- Composer (phase 48, 2026-08-29, TODO.md L3): one button, two
+           roles — #send-btn reads "Send" when idle and morphs into the
+           enabled "Stop" control (.is-stop, rose treatment) while a turn
+           is in flight; a click or Enter in flight aborts the fetch
+           (AbortController in app.js) and the partial is kept + persisted
+           with the optional `stopped` marker, rendered as the .stopped-note
+           meta-row note by app.js — live and on restore. The spinner span
+           stays in the markup (contract marker, reduced-motion pin) but
+           never shows: the Stop label + treatment carry the in-flight
+           state.
+           `novalidate`: the input is cleared after send, so a `required`
+           constraint would silently block the Stop click/Enter — the
+           browser's constraint validation runs before the `submit` event
+           and would never reach handleSend's in-flight guard. The
+           `!text` guard in app.js is the real empty-input check (same
+           precedent as the tuning form's noValidate). -->
+      <form class="composer" id="composer" novalidate>
         <label class="visually-hidden" for="message-input">Ask Brain of Reese a question</label>
         <textarea
           id="message-input"
@@ -133,7 +149,6 @@
           rows="1"
           placeholder="Ask me about the homelab…"
           autocomplete="off"
-          required
         ></textarea>
         <button type="submit" class="send-btn" id="send-btn">
           <span class="spinner" aria-hidden="true" hidden></span>