fix(chat): drop display:flex from .chat-panel-fill so grid layout wins

The inherited .chat-panel-fill class lands on ChatPanel's new .chat-full
root via attribute inheritance, and its display:flex was overriding the
grid layout — stacking messages, sidebar, and input bar vertically
inside the reading column instead of placing the sidebar in grid col 4.
Layout is ChatPanel's responsibility now; .chat-panel-fill only sizes.

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

frontend/src/views/ChatView.vue

@@ -522,12 +522,12 @@ onUnmounted(() => {
   overflow: hidden;
 }
 
-/* ChatPanel fills the remaining space in chat-main */
+/* ChatPanel fills the remaining space in chat-main. Layout (grid vs
+ * flex) is owned by ChatPanel's own .chat-full root — don't force a
+ * display here or it overrides the grid. */
 .chat-panel-fill {
   flex: 1;
   min-height: 0;
-  display: flex;
-  flex-direction: column;
 }
 
 .chat-header {