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/css/style.css

@@ -46,6 +46,7 @@
 
 * {
   box-sizing: border-box;
+  -webkit-tap-highlight-color: transparent;
 }
 
 [x-cloak] {
@@ -56,6 +57,7 @@ html,
 body {
   margin: 0;
   padding: 0;
+  overscroll-behavior-y: none;
 }
 
 body {
@@ -239,6 +241,7 @@ p {
     text-decoration: none;
     color: var(--color-text-muted);
     font-size: 11px;
+    touch-action: manipulation;
   }
 
   .bottom-tab-bar .tab-item svg {
@@ -277,6 +280,7 @@ p {
   cursor: pointer;
   font-family: inherit;
   transition: background-color 0.15s ease, opacity 0.15s ease;
+  touch-action: manipulation;
 }
 
 .btn-primary {
@@ -452,6 +456,7 @@ label {
   gap: 8px var(--space-2);
   padding: 12px 4px;
   border-bottom: 1px solid var(--color-border);
+  touch-action: manipulation;
 }
 
 .habit-item:last-child {