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/api/paypal/create/route.ts

@@ -7,6 +7,7 @@ import {
   PAYPAL_CURRENCY,
 } from "@/lib/services/paypal";
 import { env } from "@/env";
+import { logger } from "@/lib/logger";
 
 export const dynamic = "force-dynamic";
 
@@ -75,7 +76,7 @@ export async function POST(req: Request): Promise<Response> {
       credits,
     });
   } catch (e) {
-    console.error("[paypal/create]", (e as Error).message);
+    logger.error("PayPal create order failed", { module: "paypal/create", error: (e as Error).message });
     return NextResponse.json(
       { error: "Could not start the PayPal checkout. Please try again." },
       { status: 502 },