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/i18n.js

@@ -261,6 +261,8 @@
     "Put the text on the session's composer without submitting it": '将文本放入会话输入框但不提交',
     'Predicted prompt, editable': '预测的提示，可编辑',
     'Use this suggestion instead': '改用此建议',
+    "Steer the rethink, e.g. 'no, I meant the mobile bug'": '引导重想，例如："不，我是指移动端的问题"',
+    'Steer note for Rethink': '重想的引导备注',
     'Select a session first': '请先选择一个会话',
     'Read My Mind works on Claude sessions only': '读心术仅适用于 Claude 会话',
     'Prompt sent': '提示已发送',