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>

.gitignore

@@ -9,13 +9,15 @@ lerna-debug.log*
 
 # Dependencies
 node_modules
-dist
+/dist
 dist-ssr
 *.local
 
-# Frontend build output (embedded at Go build time)
-# Allow the fallback placeholder so embed always has at least index.html
-!src/dist/index.html
+# Frontend build output — embedded into the Go binary at build time.
+# Vite writes here (cmd/server/src/dist); ignore the built output but keep
+# the committed index.html placeholder so //go:embed always has a file.
+cmd/server/src/dist/*
+!cmd/server/src/dist/index.html
 
 # Environment files
 .env