feat(readmymind): rethink steer note (phase 3 part 2)

Adds the optional free-text steer note to the Read My Mind modal: a
dashed input under the suggestions ("no, I meant the mobile bug") that
rides along as `steer` on every Rethink. The API already accepted it;
this wires the frontend end of the contract.

- Shown whenever Rethink is live (ready AND empty-result phases),
  hidden only while a prediction runs; typed text survives re-runs.
- Enter in the field triggers Rethink, mirroring the prompt field's
  Enter-to-send; a fresh open clears it with the rethink memory.
- Trimmed and capped to the schema's 2000 chars on the way out; a
  plain open still sends an empty body (neither steer nor rejected).
- zh-CN strings for the placeholder and aria-label, phone-sized
  touch target in mobile.css, static guards in the phase-3 test.

Verified with a browser E2E against a live dev server (stubbed predict
endpoint): payload contents, phase visibility, Enter wiring, and
reset-on-reopen all asserted with real keystrokes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/public/index.html

@@ -2895,6 +2895,13 @@
           <div class="readmymind-alternates" id="readMyMindAlternates" data-i18n-skip style="display:none"></div>
         </div>
         <div class="readmymind-error" style="display:none">No suggestion this time. Rethink to try again.</div>
+        <!-- Rethink steer note: the user's own words about what they actually
+             meant, sent as `steer` with the next Rethink. Shown whenever
+             Rethink is live (ready AND empty-result phases), hidden while a
+             prediction runs. -->
+        <div class="readmymind-steer-row" id="readMyMindSteerRow" style="display:none">
+          <input type="text" id="readMyMindSteer" class="readmymind-steer-input" maxlength="2000" placeholder="Steer the rethink, e.g. 'no, I meant the mobile bug'" aria-label="Steer note for Rethink" onkeydown="if(event.key==='Enter')app.rethinkReadMyMind()">
+        </div>
       </div>
       <div class="modal-footer">
         <button class="btn btn-secondary" onclick="app.closeReadMyMind()">Dismiss</button>