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/lib/urlDetector.ts

@@ -109,7 +109,8 @@ export type UrlCallback = (url: string, source: UrlSource) => void;
  * A direct port of `usable_sign_in_link` in
  * `commands/auth_token_commands.rs`, and deliberately just as shallow: this is
  * a junk filter, not the security decision. `sanitizeRelayUrl` is still the
- * only thing standing between any of this and `openUrl`, and duplicating its
+ * only thing standing between any of this and `openUrlExternal`, and
+ * duplicating its
  * rules here would be a second place for them to go stale.
  *
  * The one rule from the Rust that is not ported is its `sk-ant-` check: that
@@ -293,7 +294,7 @@ export class UrlDetector {
     //    include the *whole* C0 range and DEL, not just BEL: an escape or a NUL
     //    swallowed into the middle of a match becomes a URL that renders as one
     //    thing in the toast and resolves as another. Everything emitted here is
-    //    still re-validated by `sanitizeRelayUrl` before it can reach `openUrl`;
+    //    still re-validated by `sanitizeRelayUrl` before it can reach the opener;
     //    stopping the match early only means the legitimate prefix survives
     //    instead of the whole candidate being thrown away.
     // eslint-disable-next-line no-control-regex