Refactor: Centralize template loading and improve error handling

- Introduced helpers.LoadTemplateFiles() for consistent layout + topbar rendering
- Replaced repeated template.ParseFiles() calls across handlers
- Created generic RenderError(w, r, statusCode) helper
- Replaced old Render403 with flexible RenderError
- Updated AdminOnly middleware to render 403 errors with context
- Added 500.html template for graceful panic fallback
- Prepared structure for future error codes (404, 429, etc.)

handlers/admin/dashboard.go

@@ -2,7 +2,6 @@ package handlers
 
 import (
 	"database/sql"
-	"html/template"
 	"log"
 	"net/http"
 
@@ -55,10 +54,7 @@ func AdminDashboardHandler(db *sql.DB) http.HandlerFunc {
 		}
 		context["MatchLogs"] = logs
 
-		tmpl := template.Must(template.New("").Funcs(helpers.TemplateFuncs()).ParseFiles(
-			"templates/layout.html",
-			"templates/admin/dashboard.html",
-		))
+		tmpl := helpers.LoadTemplateFiles("dashboard.html", "templates/admin/dashboard.html")
 
 		err = tmpl.ExecuteTemplate(w, "layout", context)
 		if err != nil {