feat: establish reproducible FreeCAD web compatibility baseline

public/sw.js

@@ -0,0 +1,18 @@
+const CACHE_NAME = 'bitbybit-cad-shell-v1'
+const SHELL = ['/','/index.html','/manifest.webmanifest']
+
+self.addEventListener('install', (event) => {
+  event.waitUntil(caches.open(CACHE_NAME).then((cache) => cache.addAll(SHELL)).then(() => self.skipWaiting()))
+})
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(caches.keys().then((keys) => Promise.all(keys.filter((key) => key !== CACHE_NAME).map((key) => caches.delete(key)))).then(() => self.clients.claim()))
+})
+
+self.addEventListener('fetch', (event) => {
+  if (event.request.method !== 'GET' || new URL(event.request.url).origin !== self.location.origin) return
+  event.respondWith(fetch(event.request).then((response) => {
+    if (response.ok) void caches.open(CACHE_NAME).then((cache) => cache.put(event.request, response.clone()))
+    return response
+  }).catch(() => caches.match(event.request).then((cached) => cached || caches.match('/index.html'))))
+})