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/nav-dropdown.tsx

@@ -1,5 +1,6 @@
 "use client";
 
+import Image from "next/image";
 import { useState, useRef, useEffect, type ReactNode } from "react";
 
 interface NavDropdownProps {
@@ -39,7 +40,7 @@ export function NavDropdown({ label, icon, children }: NavDropdownProps) {
         aria-expanded={open}
         aria-haspopup="true"
       >
-        {icon ? <img src={icon} alt="" className="w-5 h-5 mr-1.5" /> : null}
+        {icon ? <Image src={icon} alt="" width={20} height={20} className="mr-1.5" unoptimized /> : null}
         {label}
         <svg className={`ml-auto md:ml-1 w-3 h-3 transition-transform duration-200 ${open ? "rotate-180" : ""}`} fill="none" stroke="currentColor" viewBox="0 0 24 24"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M19 9l-7 7-7-7" /></svg>
       </button>