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/components/top-header.tsx

@@ -1,4 +1,5 @@
 import Link from "next/link";
+import Image from "next/image";
 import { auth, signOut } from "@/lib/auth";
 import { avatarImageUrl } from "@/lib/format";
 import { prisma } from "@/lib/prisma";
@@ -88,8 +89,7 @@ export async function TopHeader() {
 
             <details className="relative">
               <summary aria-label="User menu" className="list-none cursor-pointer flex items-center [&::-webkit-details-marker]:hidden">
-                {/* biome-ignore lint/performance/noImgElement: external imager */}
-                <img className="w-[54px] h-[62px] bg-no-repeat bg-center" src={avatar} alt="" />
+                <Image className="bg-no-repeat bg-center" src={avatar} alt="" width={54} height={62} unoptimized />
                 <span className="-ml-2 font-semibold" style={{ color: "var(--color-navbar-text, var(--color-text))" }}>{session.user.name}</span>
               </summary>
               <div