fix(auth): gracefully re-prompt on empty email during login

[theFong]

Problem

Running brev register (or any login flow) and hitting enter with no input at the Enter your email: prompt produced a confusing error and a raw RESTY stack trace:

Enter your email:
failed.

2026/07/01 23:32:03 WARN RESTY [error] ...maybePromptForEmail... : unexpected newline

fmt.Scanln treats an empty line as an "unexpected newline" error, which was wrapped and propagated all the way up through the auth stack.

Fix

Replace the raw fmt.Scanln call in KasAuthenticator.maybePromptForEmail with the existing terminal.PromptGetInput helper (built on promptui, already used across ~10 commands). It validates input and re-prompts automatically on empty input, so entering a blank email now shows a friendly message instead of crashing.

Notes

• The other branch of maybePromptForEmail (the "press enter to continue" confirmation) still uses fmt.Scanln, but it already tolerates the "unexpected newline" case, so it is unaffected by this bug and left unchanged to keep the diff focused.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 21.90%. Comparing base (83ace83) to head (90a82e3).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/auth/kas.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
+ Coverage   20.09%   21.90%   +1.81%     
==========================================
  Files         140      137       -3     
  Lines       18451    18511      +60     
==========================================
+ Hits         3707     4055     +348     
+ Misses      14328    14004     -324     
- Partials      416      452      +36     

Flag	Coverage Δ
Linux	21.90% <0.00%> (+1.81%)	⬆️

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