sattle: Quasar PWA scaffold with lnurlcash-kit protocol core

M1: Quasar 2 / Vue 3 / TS / Pinia / i18n / PWA app shell in the lnurl-wallet
palette (#002222/#004444/#55ffcc, Noto Sans). Protocol layer is the external
lnurlcash-kit library (git dependency on the fork until upstream merges the
prepare fix / publishes to npm); keys.ts + receive.ts vendored from
lnurl-wallet with the derivation domain renamed to 'sattle'. Nix dev shell,
ci workflow (typecheck, vitest, pwa build).

src/pages/ErrorNotFound.vue

@@ -0,0 +1,19 @@
+<template>
+  <div class="fullscreen bg-blue text-white text-center q-pa-md flex flex-center">
+    <div>
+      <div style="font-size: 30vh">404</div>
+
+      <div class="text-h2" style="opacity: 0.4">Oops. Nothing here...</div>
+
+      <q-btn
+        class="q-mt-xl"
+        color="white"
+        text-color="blue"
+        unelevated
+        to="/"
+        label="Go Home"
+        no-caps
+      />
+    </div>
+  </div>
+</template>