Add tests for root-served files and fix manifest content type

Go does not auto-detect application/manifest+json for .webmanifest
files, so set the Content-Type header explicitly. Add table-driven
tests covering status, content type, cache headers, and body for
all root-served files (favicon, manifest, robots, service-worker).

Author: pierres

internal/ui/routes.go

@@ -68,6 +68,7 @@ func handleFavicon(mux *http.ServeMux, root fs.FS) {
 
 func handleManifest(mux *http.ServeMux, root fs.FS) {
 	mux.Handle("GET /manifest.webmanifest", cacheHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/manifest+json")
 		http.ServeFileFS(w, r, root, "root/manifest.webmanifest")
 	}), staticCacheMaxAge))
 }

internal/ui/routes_test.go

@@ -0,0 +1,84 @@
+package ui
+
+import (
+	"io/fs"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"testing/fstest"
+)
+
+func TestRootFiles(t *testing.T) {
+	root := fstest.MapFS{
+		"root/favicon.ico":          &fstest.MapFile{Data: []byte("icon")},
+		"root/manifest.webmanifest": &fstest.MapFile{Data: []byte(`{"name":"test"}`)},
+		"root/robots.txt":           &fstest.MapFile{Data: []byte("User-agent: *")},
+		"root/service-worker.js":    &fstest.MapFile{Data: []byte("// sw")},
+	}
+
+	tests := []struct {
+		name        string
+		path        string
+		handler     func(*http.ServeMux, fs.FS)
+		contentType string
+		body        string
+	}{
+		{
+			name:        "favicon",
+			path:        "/favicon.ico",
+			handler:     handleFavicon,
+			contentType: "image/vnd.microsoft.icon",
+			body:        "icon",
+		},
+		{
+			name:        "manifest",
+			path:        "/manifest.webmanifest",
+			handler:     handleManifest,
+			contentType: "application/manifest+json",
+			body:        `{"name":"test"}`,
+		},
+		{
+			name:        "robots",
+			path:        "/robots.txt",
+			handler:     handleRobots,
+			contentType: "text/plain; charset=utf-8",
+			body:        "User-agent: *",
+		},
+		{
+			name:        "service-worker",
+			path:        "/service-worker.js",
+			handler:     handleServiceWorker,
+			contentType: "text/javascript; charset=utf-8",
+			body:        "// sw",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			mux := http.NewServeMux()
+			tt.handler(mux, root)
+
+			req := httptest.NewRequest(http.MethodGet, tt.path, nil)
+			rr := httptest.NewRecorder()
+			mux.ServeHTTP(rr, req)
+
+			if rr.Code != http.StatusOK {
+				t.Errorf("expected status 200, got %d", rr.Code)
+			}
+
+			ct := rr.Header().Get("Content-Type")
+			if ct != tt.contentType {
+				t.Errorf("expected Content-Type %q, got %q", tt.contentType, ct)
+			}
+
+			cc := rr.Header().Get("Cache-Control")
+			if cc == "" {
+				t.Error("expected Cache-Control header to be set")
+			}
+
+			if body := rr.Body.String(); body != tt.body {
+				t.Errorf("expected body %q, got %q", tt.body, body)
+			}
+		})
+	}
+}