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>

.gitea/workflows/ci.yaml

@@ -46,7 +46,7 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           name: dist
-          path: dist/
+          path: cmd/server/src/dist/
 
   deploy:
     needs: build