i18n(frontend): translate every remaining English string on the index page

Closes the index page's i18n coverage. Combined with the page-chrome
commit, every label users see on the dashboard is now sourced from
the TOML translation files.

Per file:
- IndexPage.vue: loading-spinner tip (initial + dynamic).
- BackupModal.vue: modal title, both list-item titles + descriptions
  ("Back up" / "Restore"), in-flight busy tips ("Importing database…"
  / "Restarting panel…").
- PanelUpdateModal.vue: modal title, update-available alert,
  current/latest version row labels, "Up to date" tag + label,
  primary action button. Modal.confirm now uses the translated
  panelUpdateDialog / panelUpdateDialogDesc with #version#
  substitution; success toast uses panelUpdateStartedPopover.
- LogModal.vue: title slot ("Logs"). The Debug/Info/Notice/Warning/
  Error log-level options stay literal — they're xray's wire values,
  not user-facing labels (matches the existing settings-page choice).
- XrayLogModal.vue: title + Filter label. Direct/Blocked/Proxy stay
  literal for the same reason.
- VersionModal.vue: modal title + xray-switch alert + per-file
  tooltip + "Update all" button + custom-geo collapse header. The
  Modal.confirm flows for switchXrayVersion + updateGeofile use
  translated dialog/desc with #version# / #filename# substitution.
- CpuHistoryModal.vue: title slot.
- CustomGeoSection.vue: routing-hint alert, Add / Update-all buttons,
  every column title (computed for live locale), copy/edit/download/
  delete tooltips, copy toast, delete-confirm modal, empty-state
  text.
- CustomGeoFormModal.vue: add/edit titles, OK/cancel labels, Type/
  Alias/URL field labels, alias placeholder, all three validation
  toasts.

Total: ~50 strings localised across 8 index-page files. The Hello /
Welcome login headline cycle and a handful of literal xray wire
values (Direct/Blocked/Proxy/log levels) are intentionally kept
hardcoded.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/pages/index/CpuHistoryModal.vue

@@ -1,8 +1,11 @@
 <script setup>
 import { ref, watch } from 'vue';
+import { useI18n } from 'vue-i18n';
 import { HttpUtil } from '@/utils';
 import Sparkline from '@/components/Sparkline.vue';
 
+const { t } = useI18n();
+
 const props = defineProps({
   open: { type: Boolean, default: false },
   status: { type: Object, required: true },
@@ -48,7 +51,7 @@ watch(bucket, () => { if (props.open) fetchBucket(); });
 <template>
   <a-modal :open="open" :closable="true" :footer="null" width="900px" @cancel="close">
     <template #title>
-      CPU history
+      {{ t('pages.index.cpu') }}
       <a-select v-model:value="bucket" size="small" class="bucket-select">
         <a-select-option :value="2">2m</a-select-option>
         <a-select-option :value="30">30m</a-select-option>