Make Crystal an installable web app with an offline shell (#39)

A manifest and icons rendered from icon.svg make the app installable.
The native build emits a service worker that precaches exactly the
built files and passes everything else to the network, so ROMs never
reach its cache. New versions wait for the player's Update, which saves
before reloading. Savegames offers installation, iPhone steps and
persistent storage, and a full disk gets its own save error.

Acceptance on physical iPhone and Android devices remains.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/styles.css

@@ -69,7 +69,22 @@ label:focus-within {
 #app {
   height: 100dvh;
   display: grid;
-  grid-template-rows: auto minmax(0, 1fr) auto;
+  /* Named rows keep the main area flexible whether or not an update
+     banner shows. */
+  grid-template-rows: auto auto minmax(0, 1fr) auto;
+  grid-template-areas: "header" "banner" "main" "bar";
+}
+#app > .site-header {
+  grid-area: header;
+}
+#app > .update-banner {
+  grid-area: banner;
+}
+#app > .app-shell {
+  grid-area: main;
+}
+#app > .global-bar {
+  grid-area: bar;
 }
 
 .site-header {
@@ -915,9 +930,34 @@ dialog h2 {
   padding-top: 20px;
   border-top: 3px double #747269;
 }
-.replace-rom h3 {
+.replace-rom h3,
+.app-settings h3 {
   margin: 0;
 }
+.app-settings {
+  max-width: 720px;
+  margin-top: 24px;
+  padding-top: 20px;
+  border-top: 3px double #747269;
+}
+.app-settings p {
+  color: #4e5157;
+}
+.app-settings .menu-button {
+  width: fit-content;
+  margin-bottom: 18px;
+}
+/* A waiting update: shown until the player chooses to reload. */
+.update-banner {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  justify-content: center;
+  gap: 6px 14px;
+  padding: 6px 12px;
+  color: var(--cream);
+  background: var(--blue);
+}
 .replace-rom .menu-button {
   width: fit-content;
 }