fix(mime): register application/manifest+json for .webmanifest (+ webp/avif); v1.7.1

Go's MIME table has no .webmanifest entry, so http.FileServer sniffed the PWA web
app manifest as text/plain. mime.AddExtensionType at package load makes TypeByExtension
authoritative. A correctness nit (the manifest spec parses by content and nosniff does
not reject manifests), but a PWA server should label its manifest correctly.

spa/mime.go

@@ -0,0 +1,24 @@
+package spa
+
+import "mime"
+
+// Go's built-in MIME table — and a container image without /etc/mime.types — has no
+// entry for these extensions, so http.FileServer falls back to content sniffing and
+// mislabels them. The one that matters for a PWA server is the web app manifest,
+// sniffed as text/plain instead of application/manifest+json. Registering the types
+// once at package load makes mime.TypeByExtension authoritative, so the correct
+// Content-Type is sent and no sniffing fallback runs.
+//
+// This is a correctness nit, not a fix for a live break: the manifest spec parses by
+// content (valid JSON) rather than declared type, so browsers accept it either way.
+// A PWA-focused server should still label it correctly. .webp/.avif get the same
+// treatment for the same reason.
+func init() {
+	for ext, typ := range map[string]string{
+		".webmanifest": "application/manifest+json",
+		".webp":        "image/webp",
+		".avif":        "image/avif",
+	} {
+		_ = mime.AddExtensionType(ext, typ)
+	}
+}