feat(i18n): add full-stack internationalization support

Add languages and entity_translations tables to database schema
Implement localized queries and translation management in backend
Integrate frontend i18n and add translation UI components

frontend/src/components/TranslationFields.vue

@@ -0,0 +1,76 @@
+<script setup lang="ts">
+import { computed } from 'vue';
+import { useI18n } from 'vue-i18n';
+import type { Language, TranslationField, TranslationMap } from '../services/api';
+
+const props = defineProps<{
+  idPrefix: string;
+  field: TranslationField;
+  label: string;
+  baseValue: string;
+  translations: TranslationMap;
+  languages: Language[];
+  required?: boolean;
+}>();
+
+const emit = defineEmits<{
+  'update:baseValue': [value: string];
+  'update:translations': [value: TranslationMap];
+}>();
+
+const { t } = useI18n();
+const visibleLanguages = computed(() => props.languages.filter((language) => language.enabled));
+const defaultLanguage = computed(() => visibleLanguages.value.find((language) => language.isDefault) ?? visibleLanguages.value[0]);
+
+function fieldValue(language: Language): string {
+  if (language.code === defaultLanguage.value?.code) {
+    return props.baseValue;
+  }
+
+  return props.translations[language.code]?.[props.field] ?? '';
+}
+
+function updateField(language: Language, value: string) {
+  if (language.code === defaultLanguage.value?.code) {
+    emit('update:baseValue', value);
+    return;
+  }
+
+  const nextTranslations: TranslationMap = { ...props.translations };
+  const nextFields = { ...(nextTranslations[language.code] ?? {}) };
+
+  if (value.trim() === '') {
+    delete nextFields[props.field];
+  } else {
+    nextFields[props.field] = value;
+  }
+
+  if (Object.keys(nextFields).length) {
+    nextTranslations[language.code] = nextFields;
+  } else {
+    delete nextTranslations[language.code];
+  }
+
+  emit('update:translations', nextTranslations);
+}
+
+function inputValue(event: Event): string {
+  return event.target instanceof HTMLInputElement ? event.target.value : '';
+}
+</script>
+
+<template>
+  <div class="translation-fields">
+    <div v-for="language in visibleLanguages" :key="language.code" class="field">
+      <label :for="`${idPrefix}-${language.code}`">
+        {{ t('common.fieldForLanguage', { field: label, language: language.name }) }}
+      </label>
+      <input
+        :id="`${idPrefix}-${language.code}`"
+        :value="fieldValue(language)"
+        :required="required && language.code === defaultLanguage?.code"
+        @input="updateField(language, inputValue($event))"
+      />
+    </div>
+  </div>
+</template>