fix(auth): don't crash when no browser can be opened

[cvince]

Problem

On a machine with no browser launcher (a VM, a container, a server over SSH), capy login printed ✓ Opened browser for authentication and then crashed:

Error: spawn xdg-open ENOENT
  ... Unhandled 'error' event

The open package starts xdg-open and returns right away; the "not found" arrives a tick later as an error event with no listener, which Node treats as fatal. The surrounding try/catch had already returned, so it never caught it. The printed login link was never usable.

Fix

• openScreen attaches an error listener to every process it starts and returns opened: boolean (read from pid, which Node leaves unset when a spawn fails).
• Login prints ✓ Opened browser for authentication only when the browser actually opened. Otherwise it prints nothing extra: the link is already on screen and the callback server keeps waiting.

Verification

• New tests in tests/ui/openScreen.test.ts cover a missing launcher (failing exactly the way Node reports it), the app-window fallback, a launcher that throws, a launcher that starts, and suppressed mode.
• Full suite: 2408 pass, 0 fail. Typecheck clean.
• Under real Node in a Linux VM with no xdg-open on PATH: the old code crashes with Unhandled 'error' event; the fixed code returns {"via":"default-browser","opened":false} and the process keeps running.