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>

static/js/app.js

@@ -162,6 +162,74 @@ function closeDetail() {
     closeMobileMenus();
 }
 
+/* ============================================
+   Custom Recurrence Builder
+   ============================================ */
+
+function toggleCustomRecurrenceBuilder(selectEl) {
+    const form = selectEl.closest('form');
+    const builder = form.querySelector('.custom-recurrence-builder');
+    if (builder) {
+        builder.classList.toggle('hidden', selectEl.value !== 'custom');
+    }
+}
+
+function toggleCustomFreqPanel(radioEl) {
+    const builder = radioEl.closest('.custom-recurrence-builder');
+    const weeklyPanel = builder.querySelector('.custom-weekly-panel');
+    const monthlyPanel = builder.querySelector('.custom-monthly-panel');
+    weeklyPanel.classList.toggle('hidden', radioEl.value !== 'weekly');
+    monthlyPanel.classList.toggle('hidden', radioEl.value !== 'monthly');
+}
+
+function assembleCustomRecurrenceRule(formEl) {
+    const recurrenceSelect = formEl.querySelector('[name="recurrence"]');
+    const ruleInput = formEl.querySelector('[name="recurrence_rule"]');
+    if (!recurrenceSelect || !ruleInput) {
+        return true;
+    }
+
+    if (recurrenceSelect.value !== 'custom') {
+        ruleInput.value = '';
+        return true;
+    }
+
+    const builder = formEl.querySelector('.custom-recurrence-builder');
+    if (!builder) {
+        return true;
+    }
+
+    const freqMode = builder.querySelector('.custom-freq-mode:checked');
+    const freq = freqMode ? freqMode.value : 'weekly';
+
+    if (freq === 'weekly') {
+        const panel = builder.querySelector('.custom-weekly-panel');
+        const interval = parseInt(panel.querySelector('.custom-interval').value, 10) || 1;
+        const days = Array.from(panel.querySelectorAll('.custom-weekday:checked')).map(cb => cb.value);
+        if (days.length === 0) {
+            ruleInput.value = '';
+            return true;
+        }
+        ruleInput.value = `FREQ=WEEKLY;INTERVAL=${interval};BYDAY=${days.join(',')}`;
+    } else {
+        const panel = builder.querySelector('.custom-monthly-panel');
+        const interval = parseInt(panel.querySelector('.custom-interval').value, 10) || 1;
+        const monthlyMode = panel.querySelector('.custom-monthly-mode:checked');
+        const mode = monthlyMode ? monthlyMode.value : 'day';
+
+        if (mode === 'day') {
+            const day = parseInt(panel.querySelector('.custom-bymonthday').value, 10) || 1;
+            ruleInput.value = `FREQ=MONTHLY;INTERVAL=${interval};BYMONTHDAY=${day}`;
+        } else {
+            const ordinal = panel.querySelector('.custom-nth-ordinal').value;
+            const weekday = panel.querySelector('.custom-nth-weekday').value;
+            ruleInput.value = `FREQ=MONTHLY;INTERVAL=${interval};BYDAY=${ordinal}${weekday}`;
+        }
+    }
+
+    return true;
+}
+
 /* ============================================
    Mobile Navigation
    ============================================ */