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/components/common/Property.tsx

@@ -1,10 +1,10 @@
-import type { FunctionComponent, ReactFragment, ReactNode } from "react";
+import type { FunctionComponent, ReactNode } from "react";
 import React from "react";
 import clsx from "clsx";
 
 type PropertyProps = {
-  title: string | ReactNode | ReactFragment;
-  children: string | ReactNode | ReactFragment;
+  title: string | ReactNode;
+  children: string | ReactNode;
   titleClass?: string;
   valueClass?: string;
 };

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;
   }
 }

src/hooks/useLookup.tsx

@@ -212,7 +212,7 @@ const useLookup = (warningHandler?: WarningHandler) => {
 
   async function getAndParse<T>(
     url: string,
-    schema: ZodSchema
+    schema: ZodSchema<T>
   ): Promise<Result<T, Error>> {
     const response = await fetch(url);
 
@@ -366,7 +366,6 @@ const useLookup = (warningHandler?: WarningHandler) => {
             )
           );
 
-        // eslint-disable-next-line @typescript-eslint/no-unsafe-assignment
         const data = await response.json();
 
         // Try each schema until one works
@@ -381,7 +380,6 @@ const useLookup = (warningHandler?: WarningHandler) => {
         );
       }
       case "json": {
-        // eslint-disable-next-line @typescript-eslint/no-unsafe-assignment
         const data = JSON.parse(target);
         for (const schema of schemas) {
           const result = schema.safeParse(data);

src/rdap.ts

@@ -796,7 +796,7 @@ const TypeValidators = new Map<
         const octet = parseInt(octets[i] ?? "", 10);
         if (isNaN(octet) || octet < 0 || octet > 255) {
           return Promise.resolve(
-            `Invalid IPv4 address: octet ${i + 1} (${octets[i]}) must be 0-255`
+            `Invalid IPv4 address: octet ${i + 1} (${octets[i] ?? 'undefined'}) must be 0-255`
           );
         }
       }