fix(ui): move chat controls to header, add new session, fix settings layout

Chat page:
- Move session selector, refresh, thinking toggle to the Header bar
  (same level as "Chat" title) instead of inside the chat content area
- Add "New Session" button (+ icon) to create fresh chat sessions
- Remove duplicate toolbar from chat body

Settings page:
- Remove max-w-2xl constraint so cards fill available width
- Redesign provider cards: compact layout with key + actions in one row
- Shorten API key display (sk-...df67 format instead of full masked key)
- Move edit/delete/star buttons inside the key row background area
- Remove duplicate "AI Providers" heading (already in card header)

src/pages/Settings/index.tsx

@@ -46,7 +46,7 @@ export function Settings() {
   };
   
   return (
-    <div className="space-y-6 max-w-2xl">
+    <div className="space-y-6 p-6">
       <div>
         <h1 className="text-2xl font-bold">Settings</h1>
         <p className="text-muted-foreground">