fix: correct @@locale values to match filenames (es-ES→es, pt-PT→pt, zh-TW→zh)

lib/l10n/arb/app_es.arb

@@ -1,5 +1,5 @@
 {
-  "@@locale": "es-ES",
+  "@@locale": "es",
   "@@last_modified": "2026-01-16",
   "appName": "SpotiFLAC",
   "@appName": {

lib/l10n/arb/app_pt.arb

@@ -1,5 +1,5 @@
 {
-  "@@locale": "pt-PT",
+  "@@locale": "pt",
   "@@last_modified": "2026-01-16",
   "appName": "SpotiFLAC",
   "@appName": {

lib/l10n/arb/app_zh.arb

@@ -1,5 +1,5 @@
 {
-  "@@locale": "zh-TW",
+  "@@locale": "zh",
   "@@last_modified": "2026-01-16",
   "appName": "SpotiFLAC",
   "@appName": {