Fix Carbon UI follow-ups

[fifthsegment]

Summary

Follow-up fixes for regressions found in the merged Carbon UI unification (#190):

1. Login buttons stacked vertically — the login form used <ButtonSet stacked>, so Clear and Sign in rendered in a column. Removed stacked so they share one row.
2. Logout showed "page not found" — logout() navigated to /login but the app state stayed authenticated, so the authenticated branch's catch-all route matched /login and rendered NotFound until a refresh. headerrightnav now dispatches a loggedout event, globalheader forwards it, and App.svelte sets state = "login" so the login page renders immediately (mirroring the existing on:authenticated flow).
3. Two "Add record" buttons in DNS custom records — the empty state rendered its own button in addition to the section-header action. The empty state now keeps only its message; the single action lives in the section header.
4. Services page content centered — Services passed narrow to PageShell, centering a 64rem column unlike other routes. Removed it so Services uses the standard page width. The not-found page intentionally stays narrow.

This PR also releases GateSentry 2.3.1: GATESENTRY_VERSION bumped in main.go with matching notes in CHANGELOG.md. Merging will tag v2.3.1 and publish the release artifacts.

Validation

• make ui-check — passed (Svelte check 0 errors/0 warnings; 15 files, 75 tests passed)
• make frontend-assets + make validate-assets — passed (embedded assets synchronized)
• make verify — passed (UI checks, asset build/validation, Go tests, Go build)
• Added regression contracts for all four fixes in the existing UI test suites

🤖 Generated with [CC]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.13%. Comparing base (f75a39b) to head (64c74dd).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #191   +/-   ##
=======================================
  Coverage   51.13%   51.13%           
=======================================
  Files         128      128           
  Lines       12669    12669           
=======================================
  Hits         6478     6478           
  Misses       5412     5412           
  Partials      779      779           

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