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/language-switcher.tsx

@@ -1,5 +1,6 @@
 "use client";
 
+import Image from "next/image";
 import { useLocale } from "next-intl";
 import { useRouter } from "next/navigation";
 import { useTransition, useState, useRef, useEffect } from "react";
@@ -45,9 +46,11 @@ export function LanguageSwitcher() {
         className="nav-item flex items-center gap-1.5 bg-transparent text-[13px]"
         style={{ border: "none", cursor: "pointer", padding: "0 0.25rem" }}
       >
-        <img
+        <Image
           src={`/assets/images/icons/flags/${current.code}.png`}
           alt={current.lang}
+          width={16}
+          height={16}
           className="inline-block h-4 w-auto"
         />
         <span>{current.code.toUpperCase()}</span>
@@ -85,9 +88,11 @@ export function LanguageSwitcher() {
                 e.currentTarget.style.color = "var(--color-text)";
               }}
             >
-              <img
+              <Image
                 src={`/assets/images/icons/flags/${l.code}.png`}
                 alt={l.lang}
+                width={16}
+                height={16}
                 className="inline-block h-4 w-auto"
               />
               {l.label}