ui(chat): fix double-scroll in web UI

Chat should scroll inside the thread, not the whole page.\n\n- Constrain the app shell to the viewport and disable outer scrolling.\n- Hide page-level scrolling for the chat tab so only .chat-thread scrolls.

ui/src/styles/layout.css

@@ -6,6 +6,8 @@
   --shell-focus-duration: 220ms;
   --shell-focus-ease: cubic-bezier(0.2, 0.85, 0.25, 1);
   min-height: 100vh;
+  height: 100vh;
+  overflow: hidden;
   display: grid;
   grid-template-columns: var(--shell-nav-width) minmax(0, 1fr);
   grid-template-rows: var(--shell-topbar-height) 1fr;
@@ -309,6 +311,11 @@
   overflow-x: hidden;
 }
 
+/* Chat handles its own scrolling (chat-thread); avoid double scrollbars. */
+.content--chat {
+  overflow: hidden;
+}
+
 .shell--chat .content {
   /* No-op: keep chat layout consistent with other tabs */
 }