feat: restore mentor page to final day/night spec

Rebuild the mentor workspace per the approved final design (day.png/night.png):
- mentorView gets a dedicated full-width immersive shell (independent 56px
  top bar with page title slot + theme mode text, hidden module-nav/market
  tape/overview/status bar) scoped to body[data-active-view=mentorView].
- Assistant messages become borderless body text with name/time above; only
  user messages use a blue bubble; keep loading/error/streaming caret states.
- 4 quick topics stay visible with history; composer restored to a framed
  ~94px card with bottom-left shortcut hint and bottom-right send button.
- Directory tools merged into one row (search + filter menu + sort); the
  filter menu still offers all/A/B/C; list selected state is an inset rounded
  fill; contact rows are borderless 75px items.
- Chat header always shows pin/note/profile/clear; pin reuses /api/mentors/
  preferences; theme toggle stays the single #themeToggle.
- Night tokens match the spec including the two distinct blues (#316FEF link,
  #5B8DEF quote/selected icon). Mobile (<768) stacks panels with no horizontal
  overflow; 1024+ follows the desktop spec.
- Update stale test baselines (300px sidebar, 94px composer, hint presence,
  night bubble color, centered disclaimer) and regenerate the architecture
  inventory.

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

frontend/shared/shell.css

@@ -3222,3 +3222,127 @@ body.sidebar-collapsed .status-bar {
     display: none;
   }
 }
+
+/* Dedicated mentor workspace (final day/night spec). Only the active mentor
+   view enables this immersive state; other pages keep the canonical shell. */
+.app-page-context {
+  min-width: 0;
+  flex: 1 1 auto;
+  flex-direction: column;
+  justify-content: center;
+  gap: 2px;
+  display: none;
+}
+
+#themeModeText {
+  display: none;
+}
+
+.app-page-context strong {
+  overflow: hidden;
+  color: var(--text-primary);
+  font-size: 18px;
+  font-weight: var(--font-weight-semibold);
+  line-height: 1.3;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.app-page-context span {
+  overflow: hidden;
+  color: var(--text-secondary);
+  font-size: 13px;
+  line-height: 1.4;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+@media (min-width: 1024px) {
+  body[data-active-view="mentorView"] .main {
+    width: 100%;
+    min-width: 0;
+    margin-left: 0;
+    height: 100dvh;
+    display: flex;
+    flex-direction: column;
+    overflow: hidden;
+  }
+
+  body[data-active-view="mentorView"] .module-nav {
+    display: none;
+  }
+
+  body[data-active-view="mentorView"] .status-bar {
+    display: none;
+  }
+
+  body[data-active-view="mentorView"] .market-tape,
+  body[data-active-view="mentorView"] .mobile-page-context {
+    display: none;
+  }
+
+  body[data-active-view="mentorView"] .overview-strip {
+    display: none;
+  }
+
+  body[data-active-view="mentorView"] .app-header {
+    flex: 0 0 56px;
+    height: 56px;
+    min-height: 56px;
+    gap: 24px;
+    padding: 0 20px;
+    background: #F5F6F7;
+  }
+
+  :root[data-theme="dark"] body[data-active-view="mentorView"] .app-header {
+    background: #141519;
+  }
+
+  body[data-active-view="mentorView"] .app-page-context {
+    display: flex;
+  }
+
+  body[data-active-view="mentorView"] .header-actions > :is(.global-search-button, .alert-button, .assistant-button, .header-menu-button) {
+    display: none;
+  }
+
+  body[data-active-view="mentorView"] .theme-toggle {
+    width: auto;
+    height: 32px;
+    min-height: 32px;
+    display: inline-flex;
+    align-items: center;
+    gap: 6px;
+    padding: 0 12px;
+    border-radius: var(--radius-md);
+    background: transparent;
+    color: var(--text-secondary);
+    font-size: var(--font-size-label);
+    font-weight: var(--font-weight-medium);
+  }
+
+  body[data-active-view="mentorView"] .theme-toggle .lucide {
+    width: 16px;
+    height: 16px;
+  }
+
+  body[data-active-view="mentorView"] #themeModeText {
+    display: inline-block;
+  }
+
+  body[data-active-view="mentorView"] .app-main {
+    flex: 1 1 auto;
+    height: calc(100dvh - 56px);
+    min-height: 0;
+    display: flex;
+    flex-direction: column;
+    padding: 0;
+    overflow: hidden;
+  }
+
+  body[data-active-view="mentorView"] .workspace-view.active-view {
+    flex: 1 1 auto;
+    min-height: 0;
+    overflow: hidden;
+  }
+}