fix(dashboard): mobile chat New Chat button placement + merged fn-2469

- ChatView mobile: hide the top sidebar header's "New Chat" button on
  <=768px and show the existing footer button as a pinned full-width
  action with safe-area padding. Both buttons existed in JSX; only
  CSS was needed.
- styles.css: comments compose/edit textareas use var(--surface) so
  they remain visible against the task detail modal's --card panel
  (in light theme --bg and --card both resolve to #ffffff).
- Includes resolved-conflict changes from fusion/fn-2469 in
  ModelOnboardingModal and SettingsModal (and the related test).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/ChatView.css

@@ -637,6 +637,12 @@
   color: var(--text-muted);
 }
 
+/* The mobile-only "New Chat" footer button is hidden on desktop; the
+   desktop layout uses the .chat-sidebar-header button instead. */
+.chat-sidebar-footer {
+  display: none;
+}
+
 /* === Mobile: one pane at a time =========================================
    Sidebar (session list) and thread are siblings of .chat-view. On mobile
    we show whichever one is "active" full-width: when the sidebar is
@@ -657,4 +663,25 @@
   .chat-sidebar:not(.chat-sidebar--hidden) + .chat-thread {
     display: none;
   }
+
+  /* On mobile, the "New Chat" affordance moves to a full-width pinned
+     footer. The desktop top-header button is hidden to free the screen
+     space for the search field and session list. */
+  .chat-sidebar-header {
+    display: none;
+  }
+
+  .chat-sidebar-footer {
+    display: block;
+    padding: var(--space-sm) var(--space-md);
+    padding-bottom: max(var(--space-sm), env(safe-area-inset-bottom, 0));
+    border-top: 1px solid var(--border);
+    background: var(--bg-secondary);
+  }
+
+  .chat-sidebar-footer .chat-sidebar-footer-btn {
+    width: 100%;
+    justify-content: center;
+    min-height: 40px;
+  }
 }