feat(caldav): add Settings UI and Event Modal CalDAV target selection

- Add CalDAV accounts card to Settings page with:
  * List of configured accounts showing URL and last sync
  * Expandable calendar list with enable/disable checkboxes
  * Sync Now, Refresh Calendars, and Delete actions per account
  * Add Account modal with name, URL, username, password fields
- Add CalDAV target selector to event modal:
  * Dropdown showing local and all enabled CalDAV calendars
  * Grouped by account using optgroups
  * Pre-selects current target when editing events
  * Includes target_caldav_account_id and target_caldav_calendar_url in save
- Add CalDAV component styles to settings.css:
  * Account cards with header, meta, and action sections
  * Expandable calendar details with checkboxes and color dots
  * Empty state for no accounts
- Add missing i18n keys for calendar enable/disable, refresh, delete confirm
- Load CalDAV targets async when modal opens
- Admin-only access to account management

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

public/styles/settings.css

@@ -772,3 +772,122 @@
   background: var(--color-surface-2);
   border-radius: var(--radius-sm);
 }
+
+/* --------------------------------------------------------
+   CalDAV Components
+   -------------------------------------------------------- */
+
+.settings-card-description {
+  font-size: var(--text-sm);
+  color: var(--color-text-secondary);
+  margin-bottom: var(--space-4);
+  line-height: 1.5;
+}
+
+.caldav-empty-state {
+  padding: var(--space-6) var(--space-4);
+  text-align: center;
+  color: var(--color-text-secondary);
+  font-size: var(--text-sm);
+}
+
+.caldav-account-item {
+  padding: var(--space-4);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-md);
+  background: var(--color-surface-2);
+  margin-bottom: var(--space-3);
+}
+
+.caldav-account-header h4 {
+  font-size: var(--text-base);
+  font-weight: var(--font-weight-semibold);
+  color: var(--color-text-primary);
+  margin: 0 0 var(--space-2);
+}
+
+.caldav-account-meta {
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-1);
+  font-size: var(--text-xs);
+  color: var(--color-text-secondary);
+}
+
+.caldav-calendars-details {
+  margin: var(--space-3) 0;
+  border-top: 1px solid var(--color-border);
+  padding-top: var(--space-3);
+}
+
+.caldav-calendars-summary {
+  cursor: pointer;
+  user-select: none;
+  font-size: var(--text-sm);
+  font-weight: var(--font-weight-medium);
+  color: var(--color-text-primary);
+  padding: var(--space-2);
+  border-radius: var(--radius-sm);
+  transition: background-color var(--transition-fast);
+}
+
+.caldav-calendars-summary:hover {
+  background-color: var(--color-surface);
+}
+
+.caldav-calendars-list {
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-2);
+  margin-top: var(--space-3);
+}
+
+.caldav-calendar-item {
+  display: flex;
+  align-items: center;
+  gap: var(--space-3);
+  padding: var(--space-2) var(--space-3);
+  border-radius: var(--radius-sm);
+  cursor: pointer;
+  transition: background-color var(--transition-fast);
+  min-height: var(--target-lg);
+}
+
+.caldav-calendar-item:hover {
+  background-color: var(--color-surface);
+}
+
+.caldav-calendar-checkbox {
+  width: 18px;
+  height: 18px;
+  accent-color: var(--color-accent);
+  cursor: pointer;
+  flex-shrink: 0;
+}
+
+.caldav-calendar-color {
+  width: 12px;
+  height: 12px;
+  border-radius: var(--radius-full);
+  flex-shrink: 0;
+}
+
+.caldav-calendar-name {
+  font-size: var(--text-sm);
+  font-weight: var(--font-weight-medium);
+  color: var(--color-text-primary);
+  flex: 1;
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.caldav-account-actions {
+  display: flex;
+  gap: var(--space-2);
+  flex-wrap: wrap;
+  margin-top: var(--space-3);
+  padding-top: var(--space-3);
+  border-top: 1px solid var(--color-border);
+}