chore: add ESLint + Prettier, fix all lint errors

- Install eslint, typescript-eslint, eslint-plugin-astro, prettier,
  prettier-plugin-astro, globals
- Add flat eslint.config.mjs with recommended rules for JS, TS, Astro
- Add .prettierrc with astro plugin
- Add lint, lint:fix, format, format:check scripts
- Fix prefer-const, no-unused-vars, no-useless-assignment across codebase
- Fix AdBanner.astro JSX fragment for Prettier compatibility
- Add eslint-disable around Yandex/Top.Mail.Ru inline scripts
- Auto-format entire codebase with Prettier

public/sw.js

@@ -1,16 +1,16 @@
-const CACHE_NAME = 'randify-v1';
+const CACHE_NAME = "randify-v1";
 
-self.addEventListener('install', (event) => {
+self.addEventListener("install", (event) => {
   self.skipWaiting();
 });
 
-self.addEventListener('activate', (event) => {
+self.addEventListener("activate", (event) => {
   event.waitUntil(self.clients.claim());
 });
 
-self.addEventListener('fetch', (event) => {
+self.addEventListener("fetch", (event) => {
   const { request } = event;
-  const isNavigate = request.mode === 'navigate';
+  const isNavigate = request.mode === "navigate";
   const isSameOrigin = new URL(request.url).origin === self.location.origin;
 
   if (!isSameOrigin) {
@@ -29,7 +29,7 @@ self.addEventListener('fetch', (event) => {
           }
           return response;
         })
-        .catch(() => caches.match(request))
+        .catch(() => caches.match(request)),
     );
   } else {
     // Cache-first for static assets (JS, CSS, SVG, fonts, etc.)
@@ -39,14 +39,18 @@ self.addEventListener('fetch', (event) => {
           return cached;
         }
         return fetch(request).then((response) => {
-          if (!response || response.status !== 200 || response.type !== 'basic') {
+          if (
+            !response ||
+            response.status !== 200 ||
+            response.type !== "basic"
+          ) {
             return response;
           }
           const clone = response.clone();
           caches.open(CACHE_NAME).then((cache) => cache.put(request, clone));
           return response;
         });
-      })
+      }),
     );
   }
 });

public/yandex_dd9dad039a139021.html

@@ -1,6 +1,8 @@
 <html>
-    <head>
-        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
-    </head>
-    <body>Verification: dd9dad039a139021</body>
+  <head>
+    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
+  </head>
+  <body>
+    Verification: dd9dad039a139021
+  </body>
 </html>