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/theme-switcher.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useEffect, useState } from "react";
+import { useState } from "react";
 
 /**
  * Light/dark toggle. The actual class lives on <html> (so `.app.dark` selectors
@@ -9,11 +9,9 @@ import { useEffect, useState } from "react";
  * theme before paint to avoid a flash — this component only reflects/toggles it.
  */
 export function ThemeSwitcher() {
-  const [dark, setDark] = useState(false);
-
-  useEffect(() => {
-    setDark(document.documentElement.classList.contains("dark"));
-  }, []);
+  const [dark, setDark] = useState(() =>
+    typeof window !== "undefined" && document.documentElement.classList.contains("dark"),
+  );
 
   function toggle() {
     const next = !dark;