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/router/library.go

@@ -38,6 +38,8 @@ func registerLibraryRoutes(cfg *Config) {
 	adminLibrary.GET("/:id/stats", cfg.LibraryHandler.GetLibraryStats)
 	adminLibrary.GET("/:id/issues/list", cfg.ProcessingIssuesHandler.ListProcessingIssues)
 	adminLibrary.GET("/:id/issues/stats", cfg.ProcessingIssuesHandler.GetProcessingIssueStats)
+	adminLibrary.POST("/:id/issues/:issueId/:mediaItemId/resolve", cfg.ProcessingIssuesHandler.ResolveProcessingIssue)
+	adminLibrary.DELETE("/:id/issues/:issueId", cfg.ProcessingIssuesHandler.DeleteProcessingIssue)
 	adminLibrary.POST("/:id/scan", func(c *echo.Context) error {
 		libraryID := c.Param("id")
 		scanReq := map[string]interface{}{