Phase 3.2: Add My Trades page

New /trades page for viewing user's Bitcoin trades:
- Upcoming trades with cancel functionality
- Trade history (completed, cancelled, no-show)
- Direction badges (BUY/SELL with colors)
- EUR ↔ BTC amounts in readable format
- Rate and premium display

Update Header navigation:
- Exchange replaces Book for regular users
- My Trades replaces Appointments

Update profile page test for new nav items.

frontend/app/profile/page.test.tsx

@@ -211,8 +211,8 @@ describe("ProfilePage - Navigation", () => {
 
     render(<ProfilePage />);
     await waitFor(() => {
-      expect(screen.getByText("Book")).toBeDefined();
-      expect(screen.getByText("Appointments")).toBeDefined();
+      expect(screen.getByText("Exchange")).toBeDefined();
+      expect(screen.getByText("My Trades")).toBeDefined();
     });
   });