Add i18n support with English and Spanish translations

- Created i18n package with translator and middleware
- Added translation files for English (en.json) and Spanish (es.json)
- Updated all HTTP handlers to use i18n for error/success messages
- Added comprehensive test coverage for i18n (87%)
- Updated CI workflow to use Go 1.24
- All tests passing with 50.8% total coverage

internal/i18n/middleware.go

@@ -0,0 +1,30 @@
+package i18n
+
+import (
+	"context"
+	"net/http"
+
+	"golang.org/x/text/language"
+)
+
+type contextKey string
+
+const languageKey contextKey = "language"
+
+func LanguageMiddleware(translator *Translator) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			acceptLanguage := r.Header.Get("Accept-Language")
+			lang := translator.GetLanguage(acceptLanguage)
+			ctx := context.WithValue(r.Context(), languageKey, lang)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}
+
+func GetLanguageFromContext(ctx context.Context) language.Tag {
+	if lang, ok := ctx.Value(languageKey).(language.Tag); ok {
+		return lang
+	}
+	return language.English
+}