PWA fixes: padded maskable icon (launcher crop), edge-to-edge bands, Firefox manifest fetch

- maskable-icon.png regenerated: logo at ~66% on the brand background —
  inside the maskable safe zone, so Android no longer crops it ('zoomed in').
- index.html: viewport-fit=cover + manifest link without
  crossorigin=use-credentials (Firefox Android fails to resolve manifest
  icons on credentialed fetches — missing launcher icon).
- custom.css: standalone-mode safe-area padding so app content clears the
  system bars; the bands show the theme-correct page background.

assets/custom.css

@@ -124,3 +124,13 @@ footer a:hover {
    area the app UI does not cover (e.g. PWA navigation-bar insets) matches. */
 html:not(.dark), html:not(.dark) body { background-color: rgb(234, 243, 234); }
 html.dark, html.dark body { background-color: rgb(34, 39, 31); }
+
+/* ---- PWA safe areas: with viewport-fit=cover the page draws under system
+   bars — the bands then show the theme-correct page background instead of
+   OS-default white, and this padding keeps app content clear of the bars. ---- */
+@media (display-mode: standalone) {
+  #root {
+    padding-top: env(safe-area-inset-top, 0px);
+    padding-bottom: env(safe-area-inset-bottom, 0px);
+  }
+}