Add custom recurrence patterns (day-of-week, nth-weekday, day-of-month)

Recurring tasks were locked to fixed daily/weekly/biweekly/monthly/yearly
intervals. The `custom` recurrence type and `recurrence_rule` field already
existed in the model and API docs, but RRULE evaluation was a TODO stub
that silently fell back to weekly, and no UI exposed the option.

Implements real RRULE parsing via dateutil.rrule, and adds a builder UI
(day-of-week checkboxes for weekly, day-of-month or Nth-weekday for
monthly) so users can express patterns like "every other Wednesday" or
"every second Tuesday" without hand-writing RRULE strings.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/tasks/task_create.html

@@ -10,7 +10,7 @@
 </div>
 
 <div style="background: var(--surface); border-radius: var(--radius-md); padding: var(--space-xl); max-width: 600px;">
-    <form method="post">
+    <form method="post" onsubmit="assembleCustomRecurrenceRule(this)">
         {% csrf_token %}
 
         <div class="form-group">
@@ -55,17 +55,7 @@
             </div>
         </div>
 
-        <div class="form-group">
-            <label class="form-label" for="recurrence">Recurrence</label>
-            <select class="form-select" id="recurrence" name="recurrence">
-                <option value="none" selected>None</option>
-                <option value="daily">Daily</option>
-                <option value="weekly">Weekly</option>
-                <option value="biweekly">Bi-weekly</option>
-                <option value="monthly">Monthly</option>
-                <option value="yearly">Yearly</option>
-            </select>
-        </div>
+        {% include 'tasks/_recurrence_fields.html' with id_prefix='' %}
 
         {% if tags %}
         <div class="form-group">