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/src/lib/__tests__/skin.test.ts

@@ -0,0 +1,69 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+const store = new Map<string, string>();
+
+async function load(search: string) {
+  vi.resetModules();
+  vi.stubGlobal("location", { search });
+  vi.stubGlobal("localStorage", {
+    getItem: (k: string) => store.get(k) ?? null,
+    setItem: (k: string, v: string) => void store.set(k, v),
+    removeItem: (k: string) => void store.delete(k),
+  });
+  return import("../skins");
+}
+
+describe("skins", () => {
+  beforeEach(() => store.clear());
+  afterEach(() => vi.unstubAllGlobals());
+
+  it("is the dolphin with nothing chosen", async () => {
+    const m = await load("");
+    expect(m.SKIN.id).toBe("dolphin");
+  });
+
+  it("takes ?theme= and remembers it", async () => {
+    const m = await load("?theme=dinosaur");
+    expect(m.SKIN.id).toBe("dinosaur");
+    expect(store.get(m.STORAGE_KEY)).toBe("dinosaur");
+  });
+
+  it("uses the remembered skin when the URL has none", async () => {
+    store.set("musicmouse.theme", "dinosaur");
+    const m = await load("?pi=1");
+    expect(m.SKIN.id).toBe("dinosaur");
+  });
+
+  it("lets ?theme= switch back", async () => {
+    store.set("musicmouse.theme", "dinosaur");
+    const m = await load("?theme=dolphin");
+    expect(m.SKIN.id).toBe("dolphin");
+    expect(store.get(m.STORAGE_KEY)).toBe("dolphin");
+  });
+
+  it("gives every skin a complete, readable game world", async () => {
+    const { SKINS } = await load("");
+    const { CREATURE_IDS } = await import("../tippen/pets");
+    const { ANIMALS } = await import("../tippen/grading");
+    for (const skin of Object.values(SKINS)) {
+      const { game } = skin;
+      for (const id of CREATURE_IDS) expect(game.pets[id].name, `${skin.id} ${id}`).not.toBe("");
+      const ladder = ANIMALS.map((animal) => game.ladder[animal.id]);
+      for (const rung of ladder) expect(rung.name && rung.emoji && rung.praise, skin.id).toBeTruthy();
+      // One look per rung: two rungs sharing a name or an emoji would read as no progress.
+      expect(new Set(ladder.map((rung) => rung.emoji)).size, skin.id).toBe(ladder.length);
+      expect(new Set(ladder.map((rung) => rung.name)).size, skin.id).toBe(ladder.length);
+      // ⭐ is the star rating, three lines below the animal on the result sheet.
+      for (const rung of ladder) expect(rung.emoji, skin.id).not.toBe("⭐");
+      for (const mode of Object.values(game.modes)) expect(mode.name && mode.emoji, skin.id).toBeTruthy();
+      expect(game.feedItems.length, skin.id).toBeGreaterThan(0);
+    }
+  });
+
+  it("ignores unknown values, in the URL and in storage", async () => {
+    store.set("musicmouse.theme", "toString");
+    const m = await load("?theme=unicorn");
+    expect(m.SKIN.id).toBe("dolphin");
+    expect(store.get(m.STORAGE_KEY)).toBe("toString");
+  });
+});