chore(frontend): make vite dev runnable outside Docker

Allow overriding the dep-optimizer cache dir via VITE_CACHE_DIR (the
default node_modules/.vite is root-owned when built in Docker) and honor
a PORT injected by the harness. Both default to prior behavior when the
env vars are unset, so Docker builds are unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -6,6 +6,9 @@ import path from 'path'
 const backendUrl = process.env.BACKEND_URL ?? 'http://localhost:8000'
 
 export default defineConfig({
+  // Allow overriding the dep-optimizer cache dir (default node_modules/.vite is
+  // owned by root when the image is built in Docker, so local `vite` can't write).
+  cacheDir: process.env.VITE_CACHE_DIR || undefined,
   plugins: [react(), tailwindcss()],
   resolve: {
     alias: {
@@ -13,6 +16,9 @@ export default defineConfig({
     },
   },
   server: {
+    // Honor a port injected by the harness (e.g. preview autoPort); falls back to
+    // Vite's default 5173 when unset.
+    port: process.env.PORT ? Number(process.env.PORT) : undefined,
     proxy: {
       '/api': {
         target: backendUrl,