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.

CHANGELOG.md

@@ -6,6 +6,17 @@ This module adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html
 
 ---
 
+## [1.7.1] — 2026-08-14
+
+Patch — correct MIME types for PWA assets.
+
+### Fixed
+
+- Register `application/manifest+json` for `.webmanifest` (plus `image/webp` and `image/avif`) at
+  startup. Without it, Go's content sniffing served the PWA web app manifest as `text/plain`.
+  Nothing broke in practice — the manifest spec parses by content, and `nosniff` does not reject
+  manifests — but a PWA-focused server should label its manifest correctly.
+
 ## [1.7.0] — 2026-08-14
 
 Minor — HTTP behaviour: caching, a strict SPA fallback, security headers, and a non-root image.