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>

test/readmymind-phone-key.test.ts

@@ -73,4 +73,20 @@ describe('read my mind phone key + alternates (static guards)', () => {
     expect(ui).toContain('readMyMindAlternates');
     expect(ui).toMatch(/\.textContent = suggestion\.prompt/);
   });
+
+  // Phase 3 part 2: the Rethink steer note (docs/readmymind-plan.md phase 3).
+  it('wires the rethink steer note end to end: field, payload, phase visibility, reset', () => {
+    // The field lives in the modal, capped to the schema's 2000-char limit,
+    // and Enter in it triggers a rethink (mirroring the prompt field's
+    // Enter-to-send).
+    expect(html).toMatch(/id="readMyMindSteer"[^>]*maxlength="2000"/);
+    expect(html).toMatch(/id="readMyMindSteer"[^>]*onkeydown="[^"]*rethinkReadMyMind\(\)"/);
+    // Predict sends the trimmed note as `steer`, bounded to the schema cap.
+    expect(ui).toMatch(/body\.steer = steer\.slice\(0, 2000\)/);
+    // The row hides ONLY during loading: Rethink is live in both the ready
+    // and the empty-result phases, so the note must be reachable in both.
+    expect(ui).toMatch(/steerRow\.style\.display = phase === 'loading' \? 'none' : ''/);
+    // A fresh open resets the note along with the rethink memory.
+    expect(ui).toMatch(/steer\.value = ''/);
+  });
 });