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/components/FuelFormModal.vue

@@ -2,6 +2,7 @@
 import { ref, computed } from "vue";
 import { api } from "../api";
 import { applyAttachment } from "../lib/attachment.js";
+import { t, tSplit } from "../i18n";
 import AttachmentField from "./AttachmentField.vue";
 import Modal from "./Modal.vue";
 
@@ -75,66 +76,64 @@ function payload() {
 </script>
 
 <template>
-  <Modal :title="isEdit ? 'Edit refill' : 'Log refill'" @close="emit('close')">
+  <Modal :title="isEdit ? t('forms.fuel.editTitle') : t('forms.fuel.addTitle')" @close="emit('close')">
     <p v-if="error" class="mb-3 rounded-control bg-danger-soft px-3 py-2 text-sm font-medium text-danger">{{ error }}</p>
     <form class="space-y-3" @submit.prevent="submit">
       <div class="grid grid-cols-2 gap-2">
         <div>
-          <label class="dh-label">Date *</label>
+          <label class="dh-label">{{ t("forms.fuel.date") }}</label>
           <input v-model="form.date" type="date" required class="dh-input data" />
         </div>
         <div>
-          <label class="dh-label">Odometer (km) *</label>
+          <label class="dh-label">{{ t("forms.fuel.odometer") }}</label>
           <input v-model="form.km" type="number" min="1" required placeholder="16138" class="dh-input data" />
         </div>
       </div>
 
       <div class="grid grid-cols-2 gap-2">
         <div>
-          <label class="dh-label">Litres *</label>
+          <label class="dh-label">{{ t("forms.fuel.liters") }}</label>
           <input v-model="form.liters" type="number" step="0.01" min="0.01" required placeholder="42.5" class="dh-input data" />
         </div>
         <div>
-          <label class="dh-label">Total cost</label>
+          <label class="dh-label">{{ t("forms.fuel.cost") }}</label>
           <input v-model="form.cost" type="number" step="0.01" min="0" placeholder="285.00" class="dh-input data" />
         </div>
       </div>
 
       <p v-if="pricePerLiter" class="text-xs text-muted">
-        Price per litre: <span class="data text-strong">{{ pricePerLiter }}</span>
+        {{ tSplit("forms.fuel.pricePerLiter", "price").before
+        }}<span class="data text-strong">{{ pricePerLiter }}</span>{{ tSplit("forms.fuel.pricePerLiter", "price").after }}
       </p>
 
       <fieldset class="rounded-control border border-subtle p-3">
-        <legend class="eyebrow px-1">Tank</legend>
+        <legend class="eyebrow px-1">{{ t("forms.fuel.tank") }}</legend>
         <label class="flex items-center gap-2 py-1 text-sm text-body">
-          <input type="checkbox" v-model="form.fullTank" class="accent-[var(--accent)]" /> Filled to full
+          <input type="checkbox" v-model="form.fullTank" class="accent-[var(--accent)]" /> {{ t("forms.fuel.fullTank") }}
         </label>
         <label class="flex items-center gap-2 py-1 text-sm text-body">
-          <input type="checkbox" v-model="form.missedFill" class="accent-[var(--accent)]" /> I missed logging a refill before this one
+          <input type="checkbox" v-model="form.missedFill" class="accent-[var(--accent)]" /> {{ t("forms.fuel.missedFill") }}
         </label>
-        <p class="mt-1.5 text-xs text-muted">
-          Consumption is measured between full tanks, so partial fills count towards the next full one.
-          Flagging a missed refill leaves that stretch out of the figures instead of reporting it as unrealistically economical.
-        </p>
+        <p class="mt-1.5 text-xs text-muted">{{ t("forms.fuel.tankHint") }}</p>
       </fieldset>
 
       <div class="grid grid-cols-2 gap-2">
         <div>
-          <label class="dh-label">Station</label>
+          <label class="dh-label">{{ t("forms.fuel.station") }}</label>
           <input v-model="form.station" placeholder="Orlen" class="dh-input" />
         </div>
         <div>
-          <label class="dh-label">Notes</label>
+          <label class="dh-label">{{ t("forms.fuel.notes") }}</label>
           <input v-model="form.notes" class="dh-input" />
         </div>
       </div>
 
-      <AttachmentField v-model:file="file" v-model:remove="removeFile" :record="entry" legend="Receipt" />
+      <AttachmentField v-model:file="file" v-model:remove="removeFile" :record="entry" :legend="t('forms.fuel.attachmentLegend')" />
 
       <div class="mt-4 flex justify-end gap-2">
-        <button type="button" class="dh-btn dh-btn-ghost" @click="emit('close')">Cancel</button>
+        <button type="button" class="dh-btn dh-btn-ghost" @click="emit('close')">{{ t("common.cancel") }}</button>
         <button type="submit" :disabled="saving" class="dh-btn dh-btn-primary">
-          {{ saving ? "Saving…" : isEdit ? "Save changes" : "Log refill" }}
+          {{ saving ? t("common.saving") : isEdit ? t("common.saveChanges") : t("forms.fuel.submit") }}
         </button>
       </div>
     </form>