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/fs/sftp.ts

@@ -3,6 +3,7 @@ import { readFileSync } from "node:fs";
 import path from "node:path";
 import { Client, type ConnectConfig, type SFTPWrapper } from "ssh2";
 import type { ConnectionConfig, EntryType, FileEntry } from "../../shared/types";
+import { t } from "../i18n";
 import { expandTilde } from "../store/ssh-config";
 import type { FileSystemProvider } from "./provider";
 
@@ -63,7 +64,7 @@ export class SftpProvider implements FileSystemProvider {
 				connectConfig.password = config.auth.password;
 				break;
 			case "agent":
-				if (!agentSock) throw new Error("ssh-agent not available (SSH_AUTH_SOCK is not set)");
+				if (!agentSock) throw new Error(t("errors.sshAgentUnavailable"));
 				connectConfig.agent = agentSock;
 				break;
 			case "key": {
@@ -73,7 +74,7 @@ export class SftpProvider implements FileSystemProvider {
 					connectConfig.privateKey = readFileSync(expandTilde(config.auth.privateKeyPath));
 				} catch {
 					if (!agentSock)
-						throw new Error(`identity file not found: ${config.auth.privateKeyPath}`);
+						throw new Error(t("errors.identityFileNotFound", { path: config.auth.privateKeyPath }));
 				}
 				if (config.auth.passphrase) connectConfig.passphrase = config.auth.passphrase;
 				else if (agentSock) connectConfig.agent = agentSock;