hscontrol: consolidate assets into single package

Move favicon.png, style.css, and headscale.svg to hscontrol/assets/
and create a single assets.go file with all embed directives.

Update hscontrol/handlers.go and hscontrol/templates/general.go to
use the centralized assets package.

hscontrol/assets/assets.go

@@ -0,0 +1,24 @@
+// Package assets provides embedded static assets for Headscale.
+// All static files (favicon, CSS, SVG) are embedded here for
+// centralized asset management.
+package assets
+
+import (
+	_ "embed"
+)
+
+// Favicon is the embedded favicon.png file served at /favicon.ico
+//
+//go:embed favicon.png
+var Favicon []byte
+
+// CSS is the embedded style.css stylesheet used in HTML templates.
+// Contains Material for MkDocs design system styles.
+//
+//go:embed style.css
+var CSS string
+
+// SVG is the embedded headscale.svg logo used in HTML templates.
+//
+//go:embed headscale.svg
+var SVG string