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

internal/handlers/processing_issues.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"bookhoard/internal/database"
+	"context"
 	"net/http"
 	"time"
 
@@ -139,3 +140,8 @@ func (h *ProcessingIssuesHandler) DeleteProcessingIssue(c *echo.Context) error {
 		"message": "Issue deleted",
 	})
 }
+
+// GetProcessingIssueStatsData returns stats for SSR (not JSON response)
+func (h *ProcessingIssuesHandler) GetProcessingIssueStatsData(ctx context.Context, libraryID pgtype.UUID) (database.GetProcessingIssueStatsRow, error) {
+	return h.db.GetProcessingIssueStats(ctx, libraryID)
+}