fix(dashboard): chat mobile single-pane, comment-box theme, ChatView FOUC

- ChatView mobile: add @media (max-width: 768px) so the session-list
  sidebar goes full-width and the thread is hidden when the sidebar
  is visible (and vice-versa). The component already had the state,
  back-button (ChevronLeft), and visibility toggling — only the CSS
  was missing.
- TaskComments compose/edit textareas now use var(--surface) for
  background. The base .spec-editor-feedback uses var(--bg), which
  in light theme resolves to #ffffff — the same as the task detail
  modal's --card panel — making the comment box invisible.
- ChatView CSS is now imported eagerly from App.tsx so it bundles
  into the main CSS file. Previously the lazy ChatView JS chunk
  loaded its CSS via an async <link> tag, producing a brief flash
  of unstyled chat UI on first render.

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

packages/dashboard/app/components/ChatView.tsx

@@ -1,4 +1,5 @@
-import "./ChatView.css";
+// ChatView.css is imported eagerly from App.tsx to avoid a flash of
+// unstyled content when the lazy chunk loads. Do not re-import here.
 import { useCallback, useEffect, useMemo, useRef, useState, type ReactNode } from "react";
 import {
   MessageSquare,