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/app/admin/favicon/favicon-form.tsx

@@ -2,6 +2,7 @@
 
 import { useRef, useState } from "react";
 import { useTranslations } from "next-intl";
+import Image from "next/image";
 import { saveFavicon, deleteFavicon } from "@/actions/save-favicon";
 import { FaviconGenerator } from "./favicon-generator";
 
@@ -70,10 +71,13 @@ export function FaviconForm({ currentUrl }: { currentUrl: string | null }) {
               {t("current")}
             </label>
             <div className="flex items-center gap-4">
-              <img
+              <Image
                 src={preview}
                 alt="Favicon preview"
-                className="w-16 h-16 rounded-lg border-2 border-[var(--border-subtle)] object-contain bg-white"
+                width={64}
+                height={64}
+                className="rounded-lg border-2 border-[var(--border-subtle)] object-contain bg-white"
+                unoptimized
               />
               <div className="text-xs text-[var(--color-text-muted)] break-all">{preview}</div>
             </div>