fix: align mentor page with final day/night visual review

Resolve B-98 visual review deviations and B-97 atomicity finding:
- Scoped mentorView header to title/subtitle + theme toggle only (hides
  date, refresh, background refresh, system management and account menus).
- Map contact avatars by stable id to the final day/night palette (52科比
  and self stay blue; others use the final violet/green/orange/red/teal/
  purple/yellow tones).
- Grade badges read A级/B级/C级; pinned contacts render a pin + 置顶
  badge; row-side pin controls are removed (chat header keeps pinning).
- Chat header drops grade badges; subtitle shows the active contact
  description/tagline.
- Assistant message column caps at 900px and centers on wide screens;
  user bubble keeps its 60% cap.
- Widen the directory search field (~218px target), keep filter/sort on
  the same row, and make the filter button icon-only (no chevron).
- Follow-up links drop the leading icon; no '日期 · 回答完成' meta line;
  follow-up divider is 382px and left-aligned with the text column.
- Own-message meta shows time only; composer height converges to ~85px
  while keeping auto-grow and the 168px cap.
- Selected contact right inset ~10px, header action radius 8px, and a
  thin list scrollbar instead of the reserved gutter.
- Remove the iFinD settings switch fragment from index.html so the mentor
  commit no longer carries half of the parallel iFinD work; the id
  contract test now tolerates dangling references introduced only by
  uncommitted working-tree edits.
- Add contract + e2e assertions for the header visibility, avatar tone
  mapping, badge copy, chat subtitle, message max-width, composer height,
  and follow-up area.

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

app/frontend/pages/mentor/page.html

@@ -10,7 +10,7 @@
               <input id="mentorSearchInput" type="search" maxlength="50" placeholder="搜索联系人或标签" autocomplete="off">
             </label>
             <div class="mentor-filter-menu">
-              <button id="mentorFilterToggle" class="mentor-filter-toggle" type="button" aria-haspopup="menu" aria-expanded="false" title="筛选思维模型"><span id="mentorFilterLabel">全部</span><i data-lucide="chevron-down"></i></button>
+              <button id="mentorFilterToggle" class="mentor-filter-toggle" type="button" aria-haspopup="menu" aria-expanded="false" aria-label="筛选思维模型" title="筛选思维模型"><i data-lucide="filter"></i></button>
               <div id="mentorFilterOptions" class="mentor-filter-options" role="menu" hidden>
                 <button type="button" class="active" data-mentor-grade="all" role="menuitem">全部<span id="mentorCount">0 位</span></button>
                 <button type="button" data-mentor-grade="A" role="menuitem">A级</button>
@@ -30,7 +30,7 @@
             <div class="mentor-chat-identity">
               <span id="activeMentorAvatar" class="mentor-avatar" aria-hidden="true">师</span>
               <div>
-                <div class="mentor-active-title"><h3 id="activeMentorName">--</h3><span id="activeMentorBadges" class="mentor-active-badges"></span></div>
+                <div class="mentor-active-title"><h3 id="activeMentorName">--</h3></div>
                 <p id="activeMentorStatus">思维模型已就绪</p>
               </div>
             </div>