Internationalization: 7 languages with live switching

Custom signal-based runtime (no i18n dependency): shared src/shared/i18n
dictionaries, ui I18nService.t() reactive by construction so language
switches live, and a backend i18n.ts for native menus + thrown errors.
Every user-facing string keyed across templates, toasts, dialogs, file
kinds, columns, macOS menu/context menus, and errors. Language picker in
Options (auto-detects OS locale, falls back to English); switching
relabels the webview and native chrome instantly. English/Spanish/French/
German fully drafted; Chinese/Japanese/Basque drafted, flagged for native
review. Docs updated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/app/transfer-bar/transfer-bar.html

@@ -1,14 +1,14 @@
 @if (transfers.expanded() && transfers.hasJobs()) {
   <div class="flex items-center justify-between border-t border-border px-3 py-1 text-[11px] text-muted-foreground">
     <span>
-      {{ transfers.activeJobs().length }} active · {{ transfers.finishedJobs().length }} finished
+      {{ i18n.t('transfer.summary', { active: transfers.activeJobs().length, finished: transfers.finishedJobs().length }) }}
     </span>
     <button
       (click)="transfers.clearFinished()"
       [disabled]="!transfers.finishedJobs().length"
       class="flex items-center gap-1 rounded px-1.5 py-0.5 hover:bg-accent hover:text-foreground disabled:opacity-40 disabled:hover:bg-transparent"
     >
-      <ng-icon name="mynaTrash" size="0.75rem" /> Clear finished
+      <ng-icon name="mynaTrash" size="0.75rem" /> {{ i18n.t('transfer.clearFinished') }}
     </button>
   </div>
   <div class="max-h-40 overflow-y-auto border-t border-border">
@@ -30,7 +30,7 @@
         @if (isActive(job)) {
           <button
             (click)="transfers.cancel(job.id)"
-            title="Cancel"
+            [title]="i18n.t('transfer.cancel')"
             class="shrink-0 rounded p-1 text-muted-foreground hover:bg-accent hover:text-destructive"
           >
             <ng-icon name="mynaX" size="0.9rem" />
@@ -38,7 +38,7 @@
         } @else {
           <button
             (click)="transfers.dismiss(job.id)"
-            title="Dismiss"
+            [title]="i18n.t('transfer.dismiss')"
             class="shrink-0 rounded p-1 text-muted-foreground hover:bg-accent hover:text-foreground"
           >
             <ng-icon name="mynaX" size="0.9rem" />