admin: pre-gzip embedded static assets, add cache headers (#9918)

The admin UI served embedded static files uncompressed and without
cache headers: embed.FS has zero mod times, so no Last-Modified, no
ETag, no 304s -- every page load re-downloaded ~700KB of css/js in
full, which gets painful over slow or tunneled links.

Gzip the static tree at generation time (go generate ./weed/admin)
and embed only the compressed mirror, shrinking the binary ~1.5MB.
The handler hands the pre-compressed bytes to gzip-capable clients,
decompresses for the rest, and sets Cache-Control, per-variant
content-hash ETags and Vary so repeat loads revalidate with a 304.
bootstrap.min.css goes 232KB -> 30KB on the wire.

A drift test keeps static_gz/ in sync with static/.

weed/admin/static_embed.go

@@ -2,13 +2,13 @@ package admin
 
 import (
 	"embed"
-	"io/fs"
 )
 
-//go:embed static/*
-var StaticFS embed.FS
+//go:generate go run gen_static_gz.go
 
-// GetStaticFS returns the embedded static filesystem
-func GetStaticFS() (fs.FS, error) {
-	return fs.Sub(StaticFS, "static")
-}
+// staticGzFS is the gzipped mirror of static/, generated by gen_static_gz.go.
+// Only the compressed copies are embedded; StaticHandler decompresses for
+// clients that do not accept gzip.
+//
+//go:embed all:static_gz
+var staticGzFS embed.FS