local-dev: Vite proxy for same-origin requests + upstream gateway Dockerfile fix

vite.config.ts now proxies `/api` and `/galaxy.gateway.v1.EdgeGateway`
to the gateway, so the browser sees only `localhost:5173` and never
trips a cross-origin preflight. `.env.development` accordingly points
`VITE_GATEWAY_BASE_URL` at the Vite origin. The proxy target is
overridable via `VITE_DEV_PROXY_TARGET=...` for non-default gateways
without touching the compose file.

`gateway/Dockerfile` previously failed to build because gateway
imports `galaxy/core` (replaced to `../ui/core` in `gateway/go.mod`)
but the Dockerfile did not copy `ui/core/` into the build context
nor declare the replace in the synthesised `go.work`. Adding both
makes `docker build -f gateway/Dockerfile .` succeed; this is the
same fix already shipped in `tools/local-dev/gateway.Dockerfile`,
back-ported to upstream.

Verified:
- docker build -f gateway/Dockerfile . — builds cleanly
- pnpm test 14/14, pnpm exec playwright test 44/44 (with CI=1 to
  force a fresh dev server; reuse keeps the previous startup env)
- curl POST through localhost:5173/api/* and /galaxy.gateway.v1.* —
  reach the gateway, no CORS preflight on the browser side

tools/local-dev/README.md updated with the new network map and the
`VITE_DEV_PROXY_TARGET` override.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

gateway/Dockerfile

@@ -1,9 +1,9 @@
 # syntax=docker/dockerfile:1.7
 
 # Build context is the workspace root (galaxy/), not the gateway/
-# subdirectory, because the gateway module pulls galaxy/{backend,model,
-# redisconn,transcoder} through the go.work replace directives. Build
-# with:
+# subdirectory, because the gateway module pulls
+# galaxy/{backend,core,model,redisconn,transcoder} through the
+# go.work replace directives. Build with:
 #
 #     docker build -t galaxy/gateway:integration -f gateway/Dockerfile .
 
@@ -23,6 +23,7 @@ COPY pkg/redisconn/  ./pkg/redisconn/
 COPY pkg/schema/     ./pkg/schema/
 COPY pkg/transcoder/ ./pkg/transcoder/
 COPY pkg/util/       ./pkg/util/
+COPY ui/core/        ./ui/core/
 COPY backend/        ./backend/
 COPY gateway/        ./gateway/
 
@@ -41,6 +42,7 @@ use (
 	./pkg/schema
 	./pkg/transcoder
 	./pkg/util
+	./ui/core
 )
 
 replace (
@@ -53,6 +55,7 @@ replace (
 	galaxy/schema v0.0.0     => ./pkg/schema
 	galaxy/transcoder v0.0.0 => ./pkg/transcoder
 	galaxy/util v0.0.0       => ./pkg/util
+	galaxy/core v0.0.0       => ./ui/core
 )
 EOF