fix(frontend): emit relative asset paths so SPA loads under any subpath (#1195)

Vite's default base of '/' baked absolute asset URLs into the built
  index.html (/assets/..., /manifest.json, /img/..., /sw-register.js),
  so any path-prefixed reverse proxy (Traefik, nginx subpath, Cloudflare
  Tunnel with path routing) served the SPA as a blank white page — the
  browser requested assets from the host root and got HTML or text/plain
  back, triggering MIME-mismatch errors on every stylesheet/script.

  Set base: '' in vite.config.ts so the HTML transform emits relative
  URLs everywhere. Update public/sw-register.js to register('sw.js')
  (relative) so SW scope auto-pins to whatever subpath the document
  loaded from.

  Out of scope: API_BASE in client.ts is still absolute. The supported
  HA embedding path remains Webpage panel + TRUSTED_FRAME_ORIGINS, not
  HA Ingress (subpath-aware SPA bootstrapping has too many failure modes
  around PWA scope, push subscriptions, and deep-link reloads to take on
  in core). Documented explicitly in docker.md.

  Reported by @Spegeli, follow-up to #1167.

frontend/public/sw-register.js

@@ -10,7 +10,7 @@ if ('serviceWorker' in navigator) {
     });
   } else {
     window.addEventListener('load', () => {
-      navigator.serviceWorker.register('/sw.js')
+      navigator.serviceWorker.register('sw.js')
         .then((registration) => {
           console.log('SW registered:', registration.scope);
         })

frontend/vite.config.ts

@@ -74,6 +74,10 @@ function serveGcodeViewer() {
 }
 
 export default defineConfig({
+  // Empty base emits relative asset URLs (./assets/... instead of /assets/...)
+  // so the built SPA loads correctly when served at any subpath — HA Ingress,
+  // nginx/Traefik path prefix, Cloudflare Tunnel path routing, etc. (#1195).
+  base: '',
   plugins: [react(), serveGcodeViewer()],
   build: {
     outDir: '../static',