Switch all user-facing time displays to 12-hour MM-DD-YYYY format

Consistently format dates and times across all templates and API
handlers using MM-DD-YYYY with 12-hour clock (03:04 PM):

- analytics.go: date keys, lastSync, lastRead timestamps
- progress.go: lastUpdated timestamp in GetAllProgress
- book_detail.templ: LastReadAt, DatePublished
- book_detail_modals.templ: progress sync timestamps, LastReadAt
- devices.templ: LastSync, LastSeen
- conflicts.templ: CreatedAt
- admin_users.templ: user CreatedAt date

templates/admin_users.templ

@@ -86,7 +86,7 @@ templ AdminUsers(currentUser User, users []User, adminCount int) {
 										</td>
 										<!-- Created -->
 										<td class="px-6 py-4 whitespace-nowrap">
-											<div class="text-sm" style="color: var(--text-secondary)">{ user.CreatedAt.Format("2006-01-02") }</div>
+											<div class="text-sm" style="color: var(--text-secondary)">{ user.CreatedAt.Format("01-02-2006") }</div>
 										</td>
 										<!-- Actions -->
 										<td class="px-6 py-4 whitespace-nowrap text-right">