fix: remove view max-width constraints; widen graph panel

- Drop max-width from .knowledge-root so the graph panel can use the full
  viewport width without hitting a cap
- Drop max-width from .chat-page (message bubbles are already self-constraining)
- Increase normal graph panel width 420px → 500px
- Increase expanded width to min(960px, 60vw) so it scales with the viewport
  and updates minichat right-offset to match

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

frontend/src/views/ChatView.vue

@@ -1034,9 +1034,6 @@ onUnmounted(() => {
   display: flex;
   height: 100%;
   overflow: hidden;
-  max-width: 1600px;
-  width: 100%;
-  margin: 0 auto;
 }
 
 .chat-sidebar {