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/Makefile

@@ -21,11 +21,17 @@ install-templ:
 
 # Generate templ files
 .PHONY: generate
-generate: install-templ
+generate: install-templ static-gz
 	@echo "Generating templ files..."
 	@templ generate ./view
 	@echo "Generated: $(TEMPL_GO_FILES)"
 
+# Regenerate the gzipped static mirror embedded into the binary
+.PHONY: static-gz
+static-gz:
+	@echo "Generating gzipped static assets..."
+	@go run gen_static_gz.go
+
 # Clean generated files
 .PHONY: clean-templ
 clean-templ: