Rebuild the admin dashboard as a Svelte 5 + TypeScript app

Frontend in web/ (Vite via bun, pure CSS, no component libraries),
built to internal/dashboard/dist and embedded into the binary; the
Dockerfile builds it in a bun stage. Old static dashboard.html removed.

internal/dashboard/embed.go

@@ -0,0 +1,22 @@
+// Package dashboard embeds the built Svelte admin dashboard from web/.
+package dashboard
+
+import (
+	"embed"
+	"io/fs"
+)
+
+// dist holds the built frontend. A committed .gitkeep keeps the embed
+// pattern valid in checkouts where `bun run build` has not run yet.
+//
+//go:embed all:dist
+var files embed.FS
+
+// FS returns the dashboard files rooted at the build output directory.
+func FS() fs.FS {
+	sub, err := fs.Sub(files, "dist")
+	if err != nil {
+		return files
+	}
+	return sub
+}