docs: update PRD with comprehensive extension features and UX integration strategy

- Added UX strategy: Extension for Quick Actions, Frontend for Management
- Organized features into 4 phases (Phase 1 completed)
- Added 14 new extension features (FR-EXT-07 to FR-EXT-17):
  * Smart Monitoring: Price alerts, whale tracking, rug pull detection
  * Trading Intelligence: Token analysis, entry/exit suggestions, portfolio tracker
  * Content Creation: Chart screenshots, AI thread generator
  * Productivity: Quick actions, notifications, keyboard shortcuts
- Added Feature Responsibility Matrix showing Extension vs Frontend roles
- Added Settings Sync strategy (FR-EXT-06) with deep links to frontend
- Documented state sync architecture: Extension ↔ Backend API ↔ Frontend

surfsense_browser_extension/sidepanel/chat/ChatHeader.tsx

@@ -0,0 +1,24 @@
+import { Settings } from "lucide-react";
+import { Button } from "@/routes/ui/button";
+
+/**
+ * Chat header with branding and settings
+ */
+export function ChatHeader() {
+    return (
+        <div className="flex items-center justify-between p-4 border-b">
+            <div className="flex items-center gap-2">
+                <img
+                    src="/assets/icon.png"
+                    alt="SurfSense"
+                    className="w-6 h-6"
+                />
+                <h1 className="font-semibold text-lg">SurfSense</h1>
+            </div>
+
+            <Button variant="ghost" size="icon">
+                <Settings className="h-4 w-4" />
+            </Button>
+        </div>
+    );
+}