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/admin/dashboard.tsx

@@ -1,5 +1,6 @@
 "use client";
 
+import Image from "next/image";
 import type { ReactNode } from "react";
 
 export interface OnlineUser {
@@ -18,7 +19,7 @@ export function StatusCard({
   label,
   value,
   hint,
-  state = "neutral",
+  state: _state = "neutral",
   icon,
 }: {
   label: string;
@@ -122,11 +123,12 @@ export function OnlineUsersWidget({
               className="flex items-center gap-3 p-3 bg-[var(--color-text-muted)]/5 rounded-lg transition-all duration-150 hover:bg-[var(--color-text-muted)]/10 hover:translate-x-0.5"
             >
               <div className="relative flex-none w-[50px] h-[62px]">
-                <img
+                <Image
                   src={`https://www.habbo.com/habbo-imaging/avatarimage?width=50&height=62&direction=2&headonly=1&crop=face&action=wav&gender=m&look=${encodeURIComponent(user.look)}`}
                   alt={user.username}
-                  className="w-full h-full rounded object-cover border border-[var(--border-subtle)]"
-                  loading="lazy"
+                  fill
+                  className="rounded object-cover border border-[var(--border-subtle)]"
+                  unoptimized
                 />
                 {user.roomId && user.roomName && (
                   <div className="absolute -bottom-0.5 -right-0.5">