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>

Web App/web/src/lib/format.js

@@ -4,6 +4,7 @@
 // odometer approaches/passes the computed next-service km.
 
 import { prefs } from "../prefs.js";
+import { t } from "../i18n/index.js";
 
 export function formatDate(value) {
   if (!value) return "—";
@@ -69,19 +70,19 @@ const STYLE = {
 // dateSignal classifies the next-due date relative to today.
 function dateSignal(nextServiceDate) {
   const days = daysUntil(nextServiceDate);
-  if (days == null) return { key: "unknown", label: "No data" };
-  if (days < 0) return { key: "overdue", label: `Service Overdue ${Math.abs(days)}d` };
-  if (days <= 30) return { key: "soon", label: `Due in ${days}d` };
-  return { key: "ok", label: `OK · ${days}d` };
+  if (days == null) return { key: "unknown", label: t("status.noData") };
+  if (days < 0) return { key: "overdue", label: t("status.serviceOverdueDays", { days: Math.abs(days) }) };
+  if (days <= 30) return { key: "soon", label: t("status.dueInDays", { days }) };
+  return { key: "ok", label: t("status.okDays", { days }) };
 }
 
 // kmSignal classifies the current odometer against the next-due km.
 function kmSignal(currentKm, nextServiceKm) {
-  if (!currentKm || !nextServiceKm) return { key: "unknown", label: "No km" };
+  if (!currentKm || !nextServiceKm) return { key: "unknown", label: t("status.noKm") };
   const remaining = nextServiceKm - currentKm;
-  if (remaining < 0) return { key: "overdue", label: `Service Overdue ${num(Math.abs(remaining))} km` };
-  if (remaining <= KM_SOON) return { key: "soon", label: `In ${num(remaining)} km` };
-  return { key: "ok", label: `${num(remaining)} km left` };
+  if (remaining < 0) return { key: "overdue", label: t("status.serviceOverdueKm", { km: num(Math.abs(remaining)) }) };
+  if (remaining <= KM_SOON) return { key: "soon", label: t("status.inKm", { km: num(remaining) }) };
+  return { key: "ok", label: t("status.kmLeft", { km: num(remaining) }) };
 }
 
 // serviceStatus combines the date- and km-based signals, returning the worse of
@@ -137,16 +138,16 @@ export function expiryStatus(doc) {
   let label;
   switch (state) {
     case "expired":
-      label = `Expired ${Math.abs(days)}d ago`;
+      label = t("status.expiredAgo", { days: Math.abs(days) });
       break;
     case "expiring_soon":
-      label = days === 0 ? "Expires today" : `Renew in ${days}d`;
+      label = days === 0 ? t("status.expiresToday") : t("status.renewInDays", { days });
       break;
     case "valid":
-      label = `Valid · ${days}d`;
+      label = t("status.validDays", { days });
       break;
     default:
-      label = "No expiry";
+      label = t("status.noExpiry");
   }
   return { key: state, label, classes: EXPIRY_STYLE[state] || EXPIRY_STYLE.no_expiry };
 }
@@ -168,19 +169,19 @@ export function reminderStatus(rem) {
   const km = rem?.kmLeft;
 
   let label;
-  if (state === "done") label = "Done";
-  else if (state === "no_trigger") label = "No trigger";
+  if (state === "done") label = t("status.done");
+  else if (state === "no_trigger") label = t("status.noTrigger");
   else if (state === "overdue") {
     const parts = [];
-    if (days != null && days < 0) parts.push(`${Math.abs(days)}d`);
-    if (km != null && km < 0) parts.push(`${num(Math.abs(km))} km`);
-    label = parts.length ? `Overdue ${parts.join(" · ")}` : "Overdue";
+    if (days != null && days < 0) parts.push(t("status.days", { days: Math.abs(days) }));
+    if (km != null && km < 0) parts.push(t("status.km", { km: num(Math.abs(km)) }));
+    label = parts.length ? t("status.overdueBy", { parts: parts.join(" · ") }) : t("status.overdue");
   } else {
     // Lead with the trigger that is closest to firing.
     const parts = [];
-    if (days != null && days >= 0) parts.push(days === 0 ? "today" : `${days}d`);
-    if (km != null && km >= 0) parts.push(`${num(km)} km`);
-    label = parts.length ? `Due in ${parts.join(" · ")}` : "Upcoming";
+    if (days != null && days >= 0) parts.push(days === 0 ? t("status.today") : t("status.days", { days }));
+    if (km != null && km >= 0) parts.push(t("status.km", { km: num(km) }));
+    label = parts.length ? t("status.dueIn", { parts: parts.join(" · ") }) : t("status.upcoming");
   }
   return { key: state, label, classes: REMINDER_STYLE[state] || REMINDER_STYLE.no_trigger };
 }