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/utils/participant.py

@@ -0,0 +1,55 @@
+"""Participant business logic utilities."""
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from src.models import Participant
+
+
+def get_active_participants(exchange_id: int) -> list["Participant"]:
+    """Get all active (non-withdrawn) participants for an exchange.
+
+    Args:
+        exchange_id: ID of the exchange
+
+    Returns:
+        List of active participants, ordered by name
+    """
+    from src.models.participant import Participant
+
+    result: list[Participant] = (
+        Participant.query.filter(
+            Participant.exchange_id == exchange_id, Participant.withdrawn_at.is_(None)
+        )
+        .order_by(Participant.name)
+        .all()
+    )
+    return result
+
+
+def is_withdrawn(participant: "Participant") -> bool:
+    """Check if participant has withdrawn.
+
+    Args:
+        participant: The participant to check
+
+    Returns:
+        True if withdrawn, False otherwise
+    """
+    return participant.withdrawn_at is not None
+
+
+def can_update_profile(participant: "Participant") -> bool:
+    """Check if participant can update their profile.
+
+    Profile updates are allowed until matching occurs.
+
+    Args:
+        participant: The participant to check
+
+    Returns:
+        True if profile updates are allowed, False otherwise
+    """
+    exchange = participant.exchange
+    allowed_states = ["draft", "registration_open", "registration_closed"]
+    return exchange.state in allowed_states