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>

src/bun/transfers/engine.ts

@@ -4,6 +4,7 @@ import { BRANDING } from "../../shared/branding";
 import type { EntryType, TransferSnapshot, TransferState } from "../../shared/types";
 import type { FileSystemProvider } from "../fs/provider";
 import type { SessionRegistry } from "../fs/registry";
+import { t } from "../i18n";
 import { walkTree } from "./walk";
 
 const PROGRESS_INTERVAL_MS = 100;
@@ -59,7 +60,7 @@ async function ensureDir(fs: FileSystemProvider, path: string): Promise<void> {
 	try {
 		const st = await fs.stat(path);
 		if (st.type === "dir") return;
-		throw new Error(`"${path}" exists and is not a directory`);
+		throw new Error(t("errors.notADirectory", { path }));
 	} catch {
 		await fs.mkdir(path);
 	}
@@ -235,7 +236,7 @@ export class TransferEngine {
 			if (sameFs) {
 				for (const file of files) {
 					if (file.srcPath === file.dstPath) {
-						throw new Error(`"${file.name}": source and destination are the same file`);
+						throw new Error(t("errors.sameFileTransfer", { name: file.name }));
 					}
 				}
 			}
@@ -343,10 +344,10 @@ export class TransferEngine {
 
 		if (samePath) {
 			throw new Error(
-				isDir ? "Cannot copy a folder onto itself" : "Source and destination are the same file",
+				isDir ? t("errors.folderOntoItself") : t("errors.sameFileNoName"),
 			);
 		}
-		throw new Error("Cannot copy a folder into itself");
+		throw new Error(t("errors.folderIntoItself"));
 	}
 
 	/**