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 ✓

eslint.config.mjs

@@ -0,0 +1,66 @@
+import js from "@eslint/js";
+import tseslint from "typescript-eslint";
+import reactHooks from "eslint-plugin-react-hooks";
+import nextPlugin from "@next/eslint-plugin-next";
+import security from "eslint-plugin-security";
+import jsxA11y from "eslint-plugin-jsx-a11y";
+import prettier from "eslint-config-prettier";
+
+export default tseslint.config(
+  js.configs.recommended,
+  ...tseslint.configs.recommended,
+  security.configs.recommended,
+  prettier,
+  {
+    plugins: {
+      "@next/next": nextPlugin,
+      "react-hooks": reactHooks,
+      "jsx-a11y": jsxA11y,
+    },
+    rules: {
+      ...nextPlugin.configs.recommended.rules,
+
+      "react-hooks/rules-of-hooks": "error",
+      "react-hooks/exhaustive-deps": "warn",
+
+      "no-console": "off",
+      "no-unused-vars": "off",
+      "@typescript-eslint/no-unused-vars": [
+        "warn",
+        { argsIgnorePattern: "^_", varsIgnorePattern: "^_" },
+      ],
+      "@typescript-eslint/no-explicit-any": "warn",
+      "@typescript-eslint/consistent-type-imports": "error",
+      "@typescript-eslint/no-non-null-assertion": "warn",
+      "prefer-const": "error",
+      "no-var": "error",
+      eqeqeq: ["error", "always", { null: "ignore" }],
+
+      "no-empty": ["warn", { allowEmptyCatch: true }],
+      "no-useless-assignment": "off",
+      "no-undef": "off",
+
+      "security/detect-object-injection": "warn",
+      "security/detect-non-literal-fs-filename": "warn",
+
+    },
+  },
+  {
+    files: ["**/*.test.ts", "**/*.test.tsx", "scripts/**"],
+    rules: {
+      "security/detect-object-injection": "off",
+      "security/detect-non-literal-fs-filename": "off",
+      "@typescript-eslint/no-explicit-any": "off",
+    },
+  },
+  {
+    ignores: [
+      ".next/",
+      "node_modules/",
+      "src/generated/",
+      "public/",
+      "prisma/migrations/",
+      "db_backup_*.sql",
+    ],
+  },
+);