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/conflicts.templ

@@ -111,7 +111,7 @@ templ Conflicts(user User, conflicts []handlers.ConflictDetailResponse, total in
 								</button>
 							</div>
 							<div class="text-sm" style="color: var(--text-secondary)">
-								Created: { conflict.CreatedAt.Format("2006-01-02 15:04:05") }
+								Created: { conflict.CreatedAt.Format("01-02-2006 03:04:05 PM") }
 								if conflict.ResolvedBy != "" {
 									| Resolved by: { conflict.ResolvedBy }
 								}