feat: implement Phase 3 participant self-management (stories 4.5, 6.1)

Implemented features:
- Story 4.5: View Participant List - participants can see other active participants
- Story 6.1: Update Profile - participants can edit name and gift ideas before matching
- Utility functions for state management and business logic
- Comprehensive unit and integration tests

New files:
- src/utils/participant.py - Business logic utilities
- src/templates/participant/profile_edit.html - Profile edit form
- tests/unit/test_participant_utils.py - Unit tests for utilities
- tests/integration/test_participant_list.py - Integration tests for participant list
- tests/integration/test_profile_update.py - Integration tests for profile updates

Modified files:
- src/routes/participant.py - Added dashboard participant list and profile edit route
- src/templates/participant/dashboard.html - Added participant list section and edit link
- src/forms/participant.py - Added ProfileUpdateForm
- src/app.py - Added participant.profile_edit to setup check exemptions
- tests/conftest.py - Added exchange_factory, participant_factory, auth_participant fixtures

All tests passing. Phase 3.3 (reminder preferences) and 3.4 (withdrawal) remain to be implemented.

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

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

src/templates/participant/dashboard.html

@@ -37,6 +37,28 @@
             <dd>{{ participant.gift_ideas }}</dd>
             {% endif %}
         </dl>
+
+        {% if can_edit_profile %}
+        <a href="{{ url_for('participant.profile_edit') }}">Edit Profile</a>
+        {% endif %}
+    </section>
+
+    <section>
+        <h2>Participants ({{ participant_count }})</h2>
+        {% if participants %}
+        <ul>
+            {% for p in participants %}
+            <li>
+                {{ p.name }}
+                {% if p.id == participant.id %}
+                <span class="badge">You</span>
+                {% endif %}
+            </li>
+            {% endfor %}
+        </ul>
+        {% else %}
+        <p>No other participants yet. Share the registration link!</p>
+        {% endif %}
     </section>
 
     <section>