feat: upgrade to Next.js 15, React 19, and migrate to ESLint 9 flat config

Major framework upgrades:
- Next.js 13 → 15.5.6
- React 18.2.0 → 19.2.0
- ESLint 8 → 9 with flat config migration

Dependency updates:
- @headlessui/react, date-fns, zod, true-myth, usehooks-ts
- @typescript-eslint packages to v8
- TypeScript to v5.9.3
- Prettier to v3.6.2

Breaking changes:
- Migrate from .eslintrc.json to eslint.config.mjs
- Remove deprecated swcMinify option from next.config.mjs
- Update all React type definitions

src/helpers.ts

@@ -134,7 +134,7 @@ export function ipv6InCIDR(ip: string, cidr: string): boolean {
     const mask = (maxMask << BigInt(128 - prefixLen)) & maxMask;
 
     return (ipInt & mask) === (rangeInt & mask);
-  } catch (e) {
+  } catch {
     return false;
   }
 }