improve PWA feel on iOS: offline fallback, overscroll, tap feedback, dark status bar, apple touch icon

Add offline.html served by the service worker when a never-visited route
is requested without network. Disable rubber-band overscroll bounce and
gray tap highlights so standalone mode feels less like a webview. Split
theme-color by prefers-color-scheme so the iOS status bar matches dark
mode, and add a proper 180x180 apple-touch-icon instead of downscaling
the 192px icon.

app/static/service-worker.js

@@ -1,4 +1,4 @@
-const CACHE_NAME = "habit-tracker-v3";
+const CACHE_NAME = "habit-tracker-v4";
 const APP_SHELL = [
   "/static/css/style.css",
   "/static/js/app.js",
@@ -9,7 +9,9 @@ const APP_SHELL = [
   "/static/js/vendor/sortable.min.js",
   "/static/icons/icon-192.png",
   "/static/icons/icon-512.png",
+  "/static/icons/icon-apple-180.png",
   "/static/manifest.json",
+  "/static/offline.html",
 ];
 
 self.addEventListener("install", (event) => {
@@ -50,7 +52,7 @@ self.addEventListener("fetch", (event) => {
           }
           return response;
         })
-        .catch(() => caches.match(request))
+        .catch(() => caches.match(request).then((cached) => cached || caches.match("/static/offline.html")))
     );
     return;
   }