refactor(admin): remove separate sidebar, integrate admin nav into main sidebar

- Delete AdminSidebar component entirely
- Add Administration section (Dashboard, Libraries, Users, Settings) to
  header sidebar, visible only for admin users
- Remove Admin Panel link from user dropdown
- Strip admin chrome (sidebar wrapper, back buttons) from all 5 admin
  page templates
- Fix activeClass to handle trailing-slash routes correctly
- Add isUserVisible helper for library visibility toggles
- Fix processing issues page: remove dead Alpine JS, wire HTMX dismiss
  with proper mediaItemId, add issue ID swap targets
- Add processing issue resolve/delete routes to library router
- Add GetProcessingIssueStatsData context-based method
- Fix users page: remove broken hx-headers auth, simplify role select
- Fix settings page: remove dead adminSettings Alpine ref

templates/utils.go

@@ -16,7 +16,7 @@ import (
 
 func activeClass(current, target string) string {
 	base := "flex items-center gap-3 px-3 py-2 rounded-lg text-sm font-medium transition-colors "
-	if current == target {
+	if strings.TrimRight(current, "/") == strings.TrimRight(target, "/") {
 		return base + "bg-brand/15 text-brand"
 	}
 	return base + "text-content-muted hover:bg-surface-hover hover:text-content"
@@ -40,6 +40,15 @@ func ContainsString(slice []string, item string) bool {
 	return false
 }
 
+func isUserVisible(userID string, visibility []UserVisibilityData) bool {
+	for _, v := range visibility {
+		if v.UserID == userID {
+			return v.IsVisible
+		}
+	}
+	return false
+}
+
 // TotalMediaCount sums the MediaCount across the given libraries,
 // used to display the total next to the "All Libraries" option.
 func TotalMediaCount(libs []LibraryData) int64 {