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/shared/shell.css

@@ -3302,7 +3302,7 @@ body.sidebar-collapsed .status-bar {
     display: flex;
   }
 
-  body[data-active-view="mentorView"] .header-actions > :is(.global-search-button, .alert-button, .assistant-button, .header-menu-button) {
+  body[data-active-view="mentorView"] .header-actions > :is(.global-search-button, .alert-button, .assistant-button, .header-menu-button, .header-date-group, .header-command-group) {
     display: none;
   }