fix: build frontend into the go:embed path so the hub binary compiles

cmd/server/main.go has //go:embed all:src/dist (relative to cmd/server/),
but Vite built to repo-root dist/, so cmd/server/src/dist never existed and
every `go build` failed with "pattern all:src/dist: no matching files found".
The hub binary has never built in CI as a result.

- vite.config.ts: outDir -> cmd/server/src/dist (emptyOutDir)
- commit cmd/server/src/dist/index.html placeholder so the embed always has
  a file (real build overwrites it)
- .gitignore: scope dist ignore to /dist; ignore cmd/server/src/dist/* but
  keep the index.html placeholder (the prior !src/dist/index.html rule
  pointed at the wrong path)
- ci.yaml: upload artifact from the new output path

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

vite.config.ts

@@ -5,6 +5,12 @@ import react from '@vitejs/plugin-react'
 // https://vite.dev/config/
 export default defineConfig({
   plugins: [react()],
+  // Build straight into the Go embed location: cmd/server/main.go has
+  // //go:embed all:src/dist relative to its package dir (cmd/server/).
+  build: {
+    outDir: 'cmd/server/src/dist',
+    emptyOutDir: true,
+  },
   server: {
     port: 3000,
     proxy: {