Web frontend: switchable dolphin/dinosaur skins

Two kids, two looks: ?theme=dolphin or ?theme=dinosaur picks one, and the
choice is remembered in the browser so the installed home-screen app keeps it.
Everything that differs per child lives in one Skin object per file under
web/src/lib/skins/; layout and feature toggles in lib/theme.ts stay shared.
The dolphin renders pixel-identical to before.

The hardcoded sea hues (210 music, 175 typing game, 340 accent) are now CSS
variables written once at startup, and the Gecko-friendly sRGB gradient stops
come from the skin. The dinosaur is jungle green with a lava-orange accent,
its own mascot, icons, manifest and browser colour.

Nothing underwater is left in the dinosaur skin: leaves instead of bubbles
(play-view canvas, room page, typing game), a walking mascot, lava blobs in
the bubble game, a hatching egg instead of the treasure chest, dino pets that
walk or fly, a prehistoric speed ladder with the T-Rex as the hero rung, and
renamed games and light colours. Pets and ladder rungs are keyed by the
backend's existing ids, used as slots, so the backend and save format are
unchanged. Pets show as emoji until drawings exist.

python-backend/.gitignore: ignore tippen-curriculum-dino.yml, the dino
device's personal curriculum, like the original.

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

web/public/dinosaur/manifest.webmanifest

@@ -0,0 +1,22 @@
+{
+  "name": "Musik Dino",
+  "short_name": "Musik",
+  "description": "Musik und Hörbücher für die MusicMouse",
+  "start_url": "/?theme=dinosaur",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#1c311c",
+  "theme_color": "#1c311c",
+  "lang": "de",
+  "icons": [
+    { "src": "/dinosaur/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/dinosaur/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    {
+      "src": "/dinosaur/icon-maskable-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}

web/public/sw.js

@@ -3,7 +3,7 @@
 // work offline - the audio, the library and the state all live on the mouse, and a
 // cached answer for any of those would be a lie.
 
-const SHELL = "musikdelphin-shell-v1";
+const SHELL = "musikdelphin-shell-v2";
 
 // There is no cover cache any more; `activate` below deletes every cache that is not
 // SHELL, which retires the two generations of cover cache this worker used to keep.
@@ -16,6 +16,7 @@ self.addEventListener("install", (event) => {
           "/",
           "/manifest.webmanifest",
           "/dolphin-mascot.png",
+          "/dinosaur-mascot.png",
           "/fonts/nunito.css",
           "/fonts/nunito-latin.woff2",
           "/fonts/nunito-latin-ext.woff2",