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 ✓

package.json

@@ -12,6 +12,8 @@
     "start": "next start",
     "prisma:generate": "prisma generate",
     "typecheck": "tsc --noEmit",
+    "lint": "eslint . --max-warnings 200",
+    "format": "prettier --write .",
     "test": "vitest run",
     "db:migrate": "tsx scripts/apply-migrations.ts",
     "db:migrate:status": "tsx scripts/apply-migrations.ts --status",
@@ -39,6 +41,8 @@
     "zod": "^3.24.0"
   },
   "devDependencies": {
+    "@eslint/js": "^10.0.1",
+    "@next/eslint-plugin-next": "^16.2.10",
     "@tailwindcss/forms": "^0.5.11",
     "@tailwindcss/postcss": "^4.3.2",
     "@tailwindcss/typography": "^0.5.20",
@@ -48,11 +52,18 @@
     "@types/react-dom": "^19.2.0",
     "@types/sanitize-html": "^2.16.1",
     "dotenv": "^16.4.0",
+    "eslint": "^10.6.0",
+    "eslint-config-prettier": "^10.1.8",
+    "eslint-plugin-jsx-a11y": "^6.10.2",
+    "eslint-plugin-react-hooks": "^7.1.1",
+    "eslint-plugin-security": "^4.0.1",
     "postcss": "^8.5.16",
+    "prettier": "^3.9.5",
     "prisma": "^7.8.0",
     "tailwindcss": "^4.3.2",
     "tsx": "^4.22.5",
     "typescript": "^5.7.0",
+    "typescript-eslint": "^8.63.0",
     "vitest": "^2.1.0"
   },
   "pnpm": {