fix: route every host-browser open through open_url_external

The Rust command existed but nothing called it. All four frontend call
sites still used `openUrl` from `@tauri-apps/plugin-opener`, so the
environment fix was inert and the three dialogs carried the same Linux bug
as the terminal: DockerInstallDialog's docs link, ClaudeAuthModal's sign-in
link and UpdateDialog's release link would all have reported success while
launching nothing.

`openUrlExternal` in tauri-commands.ts is now the single sink. There is no
platform branch: Linux gets the sanitized spawn, macOS and Windows reach
the same plugin as before but from Rust, and every platform picks up the
Rust-side re-validation, which matters because these URLs originate in an
untrusted container.

Comments in urlRelay.ts and urlDetector.ts that named `openUrl` as the sink
they guard are updated to match, and the two test files that mocked
`@tauri-apps/plugin-opener` now mock the command instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/components/settings/UpdateDialog.tsx

@@ -1,5 +1,5 @@
-import { openUrl } from "@tauri-apps/plugin-opener";
 import type { UpdateInfo } from "../../lib/types";
+import { openUrlExternal } from "../../lib/tauri-commands";
 import Modal from "../ui/Modal";
 import Button from "../ui/Button";
 import { formatBytes } from "../../lib/formatBytes";
@@ -19,7 +19,7 @@ export default function UpdateDialog({
 }: Props) {
   const handleDownload = async (url: string) => {
     try {
-      await openUrl(url);
+      await openUrlExternal(url);
     } catch (e) {
       console.error("Failed to open URL:", e);
     }