fix(ux): modal error dialog for server-refused actions + ESC closes only the topmost

- Duplicate-tag create/rename hit the uq_tags_user_name constraint and 500'd; now caught and
  returned as a clear 409 ('You already have a tag named …').
- New global error dialog (errorDialog store + ErrorDialog modal) for definitive server refusals;
  the api layer wiring + mount land with the rest of the round.
- Modal now keeps a stack so ESC dismisses only the top modal — an error over the tag editor
  closes the error and leaves the editor open.

frontend/src/i18n/locales/de/errors.json

@@ -1,4 +1,8 @@
 {
+  "title": "Nicht möglich",
+  "generic": "Der Server konnte die Aktion nicht ausführen. Bitte erneut versuchen.",
+  "server": "Serverfehler",
+  "ok": "OK",
   "boundary": {
     "title": "Etwas ist schiefgelaufen.",
     "subtitle": "Die App ist auf einen unerwarteten Fehler gestoßen.",

frontend/src/i18n/locales/en/errors.json

@@ -1,4 +1,8 @@
 {
+  "title": "Couldn't complete that",
+  "generic": "The server couldn't carry out that action. Please try again.",
+  "server": "Server error",
+  "ok": "OK",
   "boundary": {
     "title": "Something went wrong.",
     "subtitle": "The app hit an unexpected error.",

frontend/src/i18n/locales/hu/errors.json

@@ -1,4 +1,8 @@
 {
+  "title": "Nem sikerült",
+  "generic": "A szerver nem tudta végrehajtani a műveletet. Próbáld újra.",
+  "server": "Szerverhiba",
+  "ok": "OK",
   "boundary": {
     "title": "Hiba történt.",
     "subtitle": "Az alkalmazás váratlan hibába ütközött.",