panel-redesign: sign-in without an administrator follows its mockup

- panel.css: .sp-signin-form .sp-form, as theme.css has it now.
  - templates/login.html: the state is div.sp-form with the mockup's
    heading and line; the empty form of ef8a82e is gone.
  - tests: the hint is checked by its new wording, and that the page
    carries no form at all.

Evidence
  - docs/assets/panel-redesign/evidence/login-no-admin.png: 1440x900,
    headless Edge, real templates served locally (no Docker here).
    login.png re-shot after the selector change: byte-identical.
  - go test -count=1 ./internal/web/... : ok (web, auth, handlers,
    validate, view)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -71,6 +71,11 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   the component kit. Sign in names the mail host above its title; a refused
   attempt is shown as a notice above the fields. `login` and `setup` are off
   the ratchet.
+- panel-redesign (design): the sign-in page before any administrator exists
+  is a drawn screen (`login-no-admin`): the heading *No administrator yet* and
+  the pointer to the one-time setup link, in the same column as the form. The
+  signed-out column is sized through `.sp-form`, not through `form` (owner,
+  2026-10-09).
 
 ### Changed