feat: implement Story 4.1 - Access Registration Page

Allows potential participants to view exchange details and access
the registration form via unique slug URLs.

Implementation:
- Added ParticipantRegistrationForm with name, email, gift_ideas, and reminder fields
- Created GET /exchange/<slug>/register route
- Built responsive registration template with exchange details
- Exempted participant routes from admin setup requirement
- Comprehensive test coverage for all scenarios

Acceptance Criteria Met:
- Valid slug displays registration form with exchange details
- Invalid slug returns 404
- Form includes all required fields with CSRF protection
- Registration deadline is displayed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app.py

@@ -134,7 +134,16 @@ def register_setup_check(app: Flask) -> None:
         has been set up with an admin account.
         """
         # Skip check for certain endpoints
-        if request.endpoint in ["setup.setup", "static", "health"]:
+        # Participant routes are public and don't require setup
+        if request.endpoint in [
+            "setup.setup",
+            "static",
+            "health",
+            "participant.register",
+            "participant.request_access",
+            "participant.magic_login",
+            "participant.dashboard",
+        ]:
             return
 
         # Check if admin exists (always check in testing mode)