fix: reconnect mentor page into the project shell

Remove the mentor immersive rules in shared/shell.css that hid the module
nav, header actions, market tape, overview strip and status bar while
mentorView was active, which made the page look like an independent site.
Mentor now stays inside the canonical 小白复盘 shell and owns its own
"问师 + 数据日期" title header inside #mentorView (B-92 content intact).

Also fix the shell's latent 1024px overflow: .main min-width 1080 now only
applies at >=1280, and the overview strip scrolls internally instead of
clipping its right side when it is wider than the content column.

Update the contract and e2e baselines to assert the shell stays visible
and usable on mentor, page switching leaves no residue, and add a
dedicated shell-integration e2e test.

Co-authored-by: multica-agent <github@multica.ai>

app/frontend/pages/mentor/page.html

@@ -1,6 +1,12 @@
-    <section id="mentorView" class="workspace-view page member-feature-view redesigned-mentor-view mentor-page-header">
+    <section id="mentorView" class="workspace-view page member-feature-view redesigned-mentor-view">
       <div class="member-gate" hidden><div class="member-gate-icon"><i data-lucide="lock-keyhole"></i></div><div><strong>问师仅对会员开放</strong><span>开通会员后可使用思维模型进行对话。会员状态可从顶部账号标识进入。</span></div></div>
       <div id="mentorNotice" class="inline-notice" hidden></div>
+      <header class="mentor-page-header" aria-label="问师">
+        <div class="mentor-page-title">
+          <h2>问师</h2>
+          <p id="mentorPageSubtitle">与不同交易思维模型持续对话 · 数据日期 --</p>
+        </div>
+      </header>
       <div class="mentor-layout">
         <aside class="mentor-sidebar" aria-label="思维模型目录">
           <div class="mentor-directory-tools">