fix(auth): always show login URL so headless login can complete

[theFong]

Problem

brev register (and any login flow) hangs and times out after 5 minutes on a headless machine (e.g. an SSH'd server):

Enter your email: alecf@nvidia.com
Waiting for login to complete in browser...
Error: no DISPLAY environment variable specified
failed.
...
timed out waiting for login

Root cause

On a machine with no display but xdg-open installed:

1. hasBrowser() returned true — it only checked whether xdg-open existed, not whether a display was available.
2. defaultAuthFunc called browser.OpenURL(url). xdg-open printed Error: no DISPLAY environment variable specified to stderr but exited 0, so OpenURL returned nil.
3. The code took the success branch: printed "Waiting for login to complete in browser..." and returned — without ever printing the login URL.
4. With no URL to visit, login could never complete, so polling ran its full 5-minute timeout and failed.

Fix

Follow the same pattern Claude Code's CLI uses: always attempt to open the browser best-effort, but always print the login URL as a fallback, so the user is never stranded regardless of environment.

• defaultAuthFunc: attempt browser.OpenURL (ignore the result), then always showLoginURL.
• showLoginURL: reframed as a fallback — Browser didn't open? Use the URL below to sign in:.
• Discard the launcher's stderr (browser.Stderr = io.Discard) — pkg/browser pipes it to ours, which is what leaked the confusing no DISPLAY message.
• This removes the need for any platform/display detection, so hasBrowser() is deleted (net −9 lines).

Note: brev register uses an in-memory device-login flow (it does not reuse ~/.brev credentials), so it always hits this path — this fix is what unblocks register on headless machines.

Testing

• go build ./..., go vet ./pkg/auth/, go test ./pkg/auth/
• Cross-built linux/arm64 and ran on a headless (no DISPLAY) arm64 box: login now prints the URL instead of hanging.