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>

src/utils/participant.py

@@ -53,3 +53,24 @@ def can_update_profile(participant: "Participant") -> bool:
     exchange = participant.exchange
     allowed_states = ["draft", "registration_open", "registration_closed"]
     return exchange.state in allowed_states
+
+
+def can_withdraw(participant: "Participant") -> bool:
+    """Check if participant can withdraw from the exchange.
+
+    Withdrawals are only allowed before registration closes.
+    After that, admin intervention is required.
+
+    Args:
+        participant: The participant to check
+
+    Returns:
+        True if withdrawal is allowed, False otherwise
+    """
+    # Already withdrawn
+    if participant.withdrawn_at is not None:
+        return False
+
+    exchange = participant.exchange
+    allowed_states = ["draft", "registration_open"]
+    return exchange.state in allowed_states