refactor(login): Stripe-style split layout + multi-step polish

Reworks the auth flow from a single centered card into a two-pane
layout — form on the left, tenant brand on the right — and tightens the
multi-step login UX along the way. Major changes:

LAYOUT
- templates/login.phtml splits into a flex-column body so the footer
  spans both panes at the bottom instead of getting clipped under main.
- New .login-form-pane and .login-brand-pane on a 1fr/1fr grid above
  768px; mobile stacks brand on top as a slim band, form below.
- Brand pane carries a soft halo + dot grid + tinted base, all derived
  from --app-primary so it tracks the tenant accent automatically.
- Login card gets a Stripe-style hairline border + soft shadow, no
  Pico article > header sectioning band, h1 in semibold + tracking-tight.
- The "body > main" global padding is overridden for login so the brand
  panel reaches the very top + bottom edges of its column.

OS THEME FALLBACK
- New appExplicitTheme() returns the user/tenant theme or empty string,
  used by login.phtml + error.phtml to OMIT data-theme entirely when no
  preference exists. CSS prefers-color-scheme media query then drives
  the theme — DB stays the source of truth, no browser-side persistence.

MULTI-STEP UX
- Heading is stage-aware: "Login" / "Select tenant" / "Login to {tenant}".
  Drops the redundant "Login credentials" subtitle.
- Stage 3 gets an identity pill (icon + email + compact "use different
  email" button) replacing the old tenant-context block, so the user
  always sees which account they're signing in with regardless of
  multi-tenant status.
- Stage 2 tenant selection drops avatars + initials — just radio + name
  with text-overflow ellipsis for long tenant names.
- Tightens primary CTA: full-width on every stage incl. <p>-wrapped
  buttons. autofocus moves to the right input per stage (ldap_username
  / password).

NOTICE / HELP LINKS
- The placeholder "Problems logging in" link is gone (it used to point
  at the imprint route — misleading). show_support wired through 6
  auth pages and the partial removed; architecture tests adjusted.
- Help-links centered with bullet separators between items, hairline
  border-top so they read as secondary navigation under the main CTA.
- The "Encrypted / HTTPS/TLS 1.2+" trust badge at the card bottom is
  removed — modern users assume HTTPS, and the badge added noise.

DEAD CODE
- $authLogoHref, $selectedTenantAvatarUrl, $selectedTenantInitial,
  $hasSelectedTenantAvatar, $canSwitchTenant — unused after the
  identity-pill / brand-pane move, removed from all 6 auth pages.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/Support/helpers/branding.php

@@ -53,6 +53,35 @@ function currentTheme(): string
     return $theme;
 }
 
+/**
+ * Resolve the theme value to set on the html data-theme attribute.
+ *
+ * Returns an empty string when no explicit preference has been set by either
+ * the user or the tenant — callers should then OMIT data-theme entirely so
+ * the browser's prefers-color-scheme media query can drive the theme. Use
+ * this only for the html element. Internal consumers (theme toggle,
+ * logo URL resolution) keep using currentTheme() / appDefaultTheme() which
+ * always return a concrete theme string.
+ */
+function appExplicitTheme(): string
+{
+    $themes = appThemes();
+
+    if (allowUserTheme()) {
+        $userTheme = strtolower(trim((string) ($_SESSION['user']['theme'] ?? '')));
+        if ($userTheme !== '' && isset($themes[$userTheme])) {
+            return $userTheme;
+        }
+    }
+
+    $tenantTheme = strtolower(trim((string) ($_SESSION['current_tenant']['default_theme'] ?? '')));
+    if ($tenantTheme !== '' && isset($themes[$tenantTheme])) {
+        return $tenantTheme;
+    }
+
+    return '';
+}
+
 /**
  * Resolve active primary color from the current tenant.
  *