feat: implement reminder preferences and withdrawal (Stories 6.3, 6.2)

Implement Phase 3 participant self-management features:

Story 6.3 - Reminder Preferences:
- Add ReminderPreferenceForm to participant forms
- Add update_preferences route for preference updates
- Update dashboard template with reminder preference toggle
- Participants can enable/disable reminder emails at any time

Story 6.2 - Withdrawal from Exchange:
- Add can_withdraw utility function for state validation
- Create WithdrawalService to handle withdrawal process
- Add WithdrawForm with explicit confirmation requirement
- Add withdraw route with GET (confirmation) and POST (process)
- Add withdrawal confirmation email template
- Update dashboard to show withdraw link when allowed
- Withdrawal only allowed before registration closes
- Session cleared after withdrawal, user redirected to registration

All acceptance criteria met for both stories.
Test coverage: 90.02% (156 tests passing)
Linting and type checking: passed

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

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

tests/unit/test_participant_utils.py

@@ -4,6 +4,7 @@ from datetime import UTC, datetime
 
 from src.utils.participant import (
     can_update_profile,
+    can_withdraw,
     get_active_participants,
     is_withdrawn,
 )
@@ -121,3 +122,37 @@ def test_can_update_profile_completed_state(participant_factory, exchange_factor
     exchange = exchange_factory(state="completed")
     participant = participant_factory(exchange=exchange)
     assert can_update_profile(participant) is False
+
+
+def test_can_withdraw_draft_state(participant_factory, exchange_factory):
+    """Withdrawal allowed in draft state."""
+    exchange = exchange_factory(state="draft")
+    participant = participant_factory(exchange=exchange)
+    assert can_withdraw(participant) is True
+
+
+def test_can_withdraw_registration_open(participant_factory, exchange_factory):
+    """Withdrawal allowed when registration open."""
+    exchange = exchange_factory(state="registration_open")
+    participant = participant_factory(exchange=exchange)
+    assert can_withdraw(participant) is True
+
+
+def test_can_withdraw_registration_closed(participant_factory, exchange_factory):
+    """Withdrawal blocked when registration closed."""
+    exchange = exchange_factory(state="registration_closed")
+    participant = participant_factory(exchange=exchange)
+    assert can_withdraw(participant) is False
+
+
+def test_can_withdraw_matched_state(participant_factory, exchange_factory):
+    """Withdrawal blocked after matching."""
+    exchange = exchange_factory(state="matched")
+    participant = participant_factory(exchange=exchange)
+    assert can_withdraw(participant) is False
+
+
+def test_can_withdraw_already_withdrawn(participant_factory):
+    """Withdrawal blocked if already withdrawn."""
+    participant = participant_factory(withdrawn_at=datetime.now(UTC))
+    assert can_withdraw(participant) is False