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/templates/participant/dashboard.html

@@ -61,6 +61,34 @@
         {% endif %}
     </section>
 
+    <section>
+        <h2>Email Reminders</h2>
+        <form method="POST" action="{{ url_for('participant.update_preferences') }}">
+            {{ reminder_form.hidden_tag() }}
+            <div>
+                {{ reminder_form.reminder_enabled() }}
+                {{ reminder_form.reminder_enabled.label }}
+            </div>
+            {% if reminder_form.reminder_enabled.description %}
+            <small>{{ reminder_form.reminder_enabled.description }}</small>
+            {% endif %}
+            <button type="submit">Update Preferences</button>
+        </form>
+    </section>
+
+    {% if can_withdraw %}
+    <section>
+        <h2>Withdraw from Exchange</h2>
+        <p>
+            If you can no longer participate, you can withdraw from this exchange.
+            This cannot be undone.
+        </p>
+        <a href="{{ url_for('participant.withdraw') }}" role="button" class="secondary">
+            Withdraw from Exchange
+        </a>
+    </section>
+    {% endif %}
+
     <section>
         <form method="POST" action="{{ url_for('participant.logout') }}">
             <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">

src/templates/participant/withdraw.html

@@ -0,0 +1,47 @@
+{% extends "layouts/base.html" %}
+
+{% block title %}Withdraw from {{ exchange.name }}{% endblock %}
+
+{% block content %}
+<article>
+    <header>
+        <h1>Withdraw from Exchange</h1>
+    </header>
+
+    <div role="alert" style="background-color: #fff3cd; border-left: 4px solid #ffc107; padding: 1rem; margin: 1rem 0;">
+        <h2 style="margin-top: 0;">⚠️ Are you sure?</h2>
+        <p>Withdrawing from this exchange means:</p>
+        <ul>
+            <li>Your registration will be cancelled</li>
+            <li>You will be removed from the participant list</li>
+            <li>You cannot undo this action</li>
+            <li>You will need to re-register with a different email to rejoin</li>
+        </ul>
+    </div>
+
+    <form method="POST" action="{{ url_for('participant.withdraw') }}">
+        {{ form.hidden_tag() }}
+
+        <div>
+            <label>
+                {{ form.confirm() }}
+                {{ form.confirm.label.text }}
+            </label>
+            {% if form.confirm.errors %}
+            <ul style="color: #dc3545; list-style: none; padding: 0;">
+                {% for error in form.confirm.errors %}
+                <li>{{ error }}</li>
+                {% endfor %}
+            </ul>
+            {% endif %}
+        </div>
+
+        <div>
+            {{ form.submit(class="contrast") }}
+            <a href="{{ url_for('participant.dashboard', id=exchange.id) }}" role="button" class="secondary">
+                Cancel
+            </a>
+        </div>
+    </form>
+</article>
+{% endblock %}