fix(ui): make animated background drift subtly

The animated background had an animation assigned, but the layers painted at the
viewport size so the background-position changes were effectively static. Give
the ambient light layers larger paint areas and drift them slowly so the
animated option visibly moves without becoming distracting.

Reduced-motion users keep the same static background.

Test Plan:
- git diff --check

Refs: user redesign nitpick about animated background not moving

crates/lanspread-tauri-deno-ts/src/styles/launcher.css

@@ -36,15 +36,23 @@
       transparent 55%
     ),
     linear-gradient(180deg, #0c1218 0%, var(--bg-0) 100%);
-  background-size: 100% 100%;
-  animation: bgshift 18s ease-in-out infinite alternate;
+  background-size: 145% 130%, 140% 125%, 100% 100%;
+  animation: bgshift 22s ease-in-out infinite alternate;
 }
 @keyframes bgshift {
   0% {
-    background-position: 0% 0%, 0% 0%, 0% 0%;
+    background-position: 0% 0%, 100% 0%, 0% 0%;
+  }
+  50% {
+    background-position: 12% 6%, 82% 9%, 0% 0%;
   }
   100% {
-    background-position: 10% 4%, -6% 2%, 0% 0%;
+    background-position: 4% 10%, 92% 16%, 0% 0%;
+  }
+}
+@media (prefers-reduced-motion: reduce) {
+  .bg-animated {
+    animation: none;
   }
 }