Add contact antispam and fix gallery video playback.

English-only messages, rate limiting, min fill time, and normalized email
validation; improve modal video serving with posters, correct MIME types, and
no gzip on gallery media.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/templates/helpers.go

@@ -1,7 +1,22 @@
 package templates
 
-import "time"
+import (
+	"path/filepath"
+	"strings"
+	"time"
+)
 
 func currentYear() string {
 	return time.Now().Format("2006")
 }
+
+func videoMIME(filename string) string {
+	switch strings.ToLower(filepath.Ext(filename)) {
+	case ".webm":
+		return "video/webm"
+	case ".mov":
+		return "video/quicktime"
+	default:
+		return "video/mp4"
+	}
+}