Security hardening, code quality, and ESLint setup

- Remove production DB dump (db_backup_*.sql) and update.log from git tracking
- Add DB backups to .gitignore
- Replace all console.log/console.error with structured logger module
- Translate Dutch error messages to English (link-discord.ts)
- Remove dead code blocks (register-form.tsx false && pattern)
- Add ESLint flat config with TypeScript, React, Next.js, jsx-a11y, and security plugins
- Add Prettier config
- Add eslint-plugin-security for security-aware linting
- Fix all 119+ ESLint warnings across the codebase:
  - Resolve security/detect-object-injection with safe access patterns
  - Resolve security/detect-non-literal-fs-filename with path traversal validation
  - Replace <img> with next/image <Image> component
  - Remove unused variables and imports
  - Replace non-null assertions with proper type guards
  - Replace <a> with <Link> for internal navigation
  - Use next/script Script component for external scripts
- Fix setState-in-useEffect anti-patterns (navbar-color-picker, logo-generator, theme-switcher)
- Add lint and format scripts to package.json

All checks: typecheck ✓, tests 58/58 ✓, lint 0 errors 0 warnings ✓

src/app/admin/layout.tsx

@@ -1,3 +1,4 @@
+import Link from "next/link";
 import { redirect } from "next/navigation";
 import type { ReactNode } from "react";
 import { getTranslations } from "next-intl/server";
@@ -90,9 +91,6 @@ function getNavGroups(t: (key: string) => string) {
 
 export default async function AdminLayout({ children }: { children: ReactNode }) {
   const staff = await requireStaff();
-  const t = await getTranslations("pages.admin.nav");
-  const navGroups = getNavGroups(t);
-
   if (await siteSettings.getBool("force_staff_2fa", false)) {
     const u = await prisma.user
       .findUnique({ where: { id: staff.id }, select: { twoFactorConfirmedAt: true } })
@@ -147,13 +145,13 @@ async function Sidebar({ staff }: { staff: { id: number; username: string; rank:
       </nav>
 
       <div className="px-3 py-3 border-t border-white/[0.06]">
-        <a
+        <Link
           href="/"
           className="flex items-center gap-2.5 px-2.5 py-2 rounded-lg text-[#c8cbe0]/60 text-xs font-medium hover:text-white hover:bg-white/5 transition-all duration-150 no-underline"
         >
           <LogOut size={14} />
           <span>{t("backToSite")}</span>
-        </a>
+        </Link>
       </div>
     </aside>
   );