feat: constrain Chat/Knowledge width + persistent expandable graph panel

Layout:
- Chat and Knowledge views now max out at 1600px and center on wide
  screens, consistent with the rest of the app; app-content overflow
  is set to hidden for both so they manage their own scroll

Graph panel (Knowledge):
- Open/closed state persisted to localStorage (fa_knowledge_graph_open)
  — stays open across navigation and page refreshes
- Expanded state persisted (fa_knowledge_graph_expanded): chevron button
  in the panel header toggles between 420px (normal) and 700px (expanded)
- Minichat right offset follows the panel width with a matching transition

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.vue

@@ -318,7 +318,9 @@ onUnmounted(() => {
   min-height: 0;
   overflow-y: auto;
 }
-.app-content:has(.workspace-root) {
+.app-content:has(.workspace-root),
+.app-content:has(.chat-page),
+.app-content:has(.knowledge-root) {
   overflow: hidden;
 }