Add a language-switch system with per-language files

Introduce a hand-rolled i18n layer across all three UIs, each reading its
text from per-language JSON files (English base + Polish + Danish). Nothing
in the converted screens hardcodes English any more.

- Web App (Vue): src/i18n/{en,pl,da}.json + index.js exposing t()/tSplit(),
  reactive to the signed-in profile locale. Every view, component, form and
  the status labels in lib/format.js go through t().
- API Server panel (Vue): src/i18n/ with its own localStorage-persisted
  language (the panel has no user profile) and a header language picker.
  Chrome, cards, login and API section titles translated; endpoint reference
  descriptions intentionally kept in English. Rebuilt embedded dist.
- Phone App (Flutter): assets/i18n/ + lib/i18n.dart loaded at startup,
  driven by AppSettings.locale. Nav, login, lock, dashboard, the full
  Settings panel (incl. language picker) and format.dart status labels
  translated; remaining detail screens fall back to English.

Language = the language half of the existing BCP-47 locale; the region half
still drives date/number/currency formatting. Missing keys fall back to
English, and plurals use Intl.PluralRules / Intl.plural so Polish gets the
correct one/few/many forms. Settings flags languages without a translation.

Tests updated to assert the localized (Polish) status wording; all pass.
See TRANSLATIONS.md for the format and how to add a language.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Phone App/test/models_format_test.dart

@@ -9,12 +9,20 @@
 import "package:flutter_test/flutter_test.dart";
 import "package:intl/date_symbol_data_local.dart";
 import "package:carcontrol_phone/format.dart";
+import "package:carcontrol_phone/i18n.dart";
 import "package:carcontrol_phone/main.dart";
 import "package:carcontrol_phone/models.dart";
 
 void main() {
+  // rootBundle (used by loadTranslations) needs the binding initialised.
+  TestWidgetsFlutterBinding.ensureInitialized();
+
   setUpAll(() async {
     await initializeDateFormatting();
+    await loadTranslations();
+    // Polish exercises both the region-aware number grouping and the localized
+    // status wording (one/few/many plurals), so the badge labels below are the
+    // Polish strings — the same text the web app renders under pl.
     appSettings.locale = "pl-PL";
     appSettings.currency = "PLN";
     appSettings.dateFormat = "DMY";
@@ -67,11 +75,11 @@ void main() {
           "expiry": {"state": state, "daysUntilExpiry": days},
           "hasFile": false,
         });
-    expect(expiryStatus(doc("expired", -5).expiry).label, "Expired 5d ago");
-    expect(expiryStatus(doc("expiring_soon", 0).expiry).label, "Expires today");
-    expect(expiryStatus(doc("expiring_soon", 12).expiry).label, "Renew in 12d");
-    expect(expiryStatus(doc("valid", 200).expiry).label, "Valid · 200d");
-    expect(expiryStatus(doc("no_expiry", null).expiry).label, "No expiry");
+    expect(expiryStatus(doc("expired", -5).expiry).label, "Wygasło 5 dni temu");
+    expect(expiryStatus(doc("expiring_soon", 0).expiry).label, "Wygasa dzisiaj");
+    expect(expiryStatus(doc("expiring_soon", 12).expiry).label, "Odnowienie za 12 dni");
+    expect(expiryStatus(doc("valid", 200).expiry).label, "Ważne · 200 dni");
+    expect(expiryStatus(doc("no_expiry", null).expiry).label, "Bezterminowe");
     expect(expiryStatus(doc("expired", -5).expiry).key, StatusKey.overdue);
   });
 
@@ -79,16 +87,19 @@ void main() {
     Reminder rem(Map<String, dynamic> extra) =>
         Reminder.fromJson({"id": "r", "car": "c", "title": "t", "type": "service", ...extra});
 
-    expect(reminderStatus(rem({"status": "done", "done": true})).label, "Done");
-    expect(reminderStatus(rem({"status": "no_trigger"})).label, "No trigger");
+    expect(reminderStatus(rem({"status": "done", "done": true})).label, "Gotowe");
+    expect(reminderStatus(rem({"status": "no_trigger"})).label, "Brak wyzwalacza");
     expect(
         reminderStatus(rem({"status": "overdue", "daysLeft": -3, "kmLeft": -200})).label,
-        "Overdue 3d · 200 km");
-    expect(reminderStatus(rem({"status": "due_soon", "daysLeft": 0})).label, "Due in today");
-    // The km count is grouped per the chosen locale (pl-PL uses a space), which
-    // is the whole point of routing every number through the one helper.
-    expect(reminderStatus(rem({"status": "upcoming", "daysLeft": 40, "kmLeft": 5000})).label,
-        "Due in 40d · 5 000 km");
+        "Zaległe 3 dni · 200 km");
+    expect(reminderStatus(rem({"status": "due_soon", "daysLeft": 0})).label, "Termin za dzisiaj");
+    // The km count is grouped per the chosen locale (pl-PL groups thousands with
+    // a space), which is the whole point of routing every number through the one
+    // helper. Asserted as start + end so the exact space glyph (ICU uses a
+    // non-breaking space) does not make the test brittle.
+    final due = reminderStatus(rem({"status": "upcoming", "daysLeft": 40, "kmLeft": 5000})).label;
+    expect(due, startsWith("Termin za 40 dni · 5"));
+    expect(due, endsWith("000 km"));
   });
 
   test("TechnicalCheck: a failed check derives no next date", () {
@@ -121,7 +132,7 @@ void main() {
       "hasFile": false,
     });
     expect(m.totalCost, 150.0);
-    expect(warrantyStatus(m)!.label, "Warranty ends in 10d");
+    expect(warrantyStatus(m)!.label, "Gwarancja kończy się za 10 dni");
     expect(warrantyStatus(m)!.key, StatusKey.soon);
 
     final noWarranty = MaintenanceEntry.fromJson(