fix(auth): restore dark split-layout visuals on /login /register

Three regressions from Task 7-9 caused /login /register to render broken:
- bg-glow not rendering: puro.css scopes .bg-glow to .puro-page,
  AuthLayout isn't inside one. Fix: duplicate bg-glow rules into
  AuthLayout scoped CSS keyed on .auth-shell-split.
- .auth-main had no background: right side showed naked body bg.
  Fix: .auth-shell-split now sets var(--bg-0) for whole shell.
- Heading/label colors used text-gray-900 light-mode classes,
  invisible on dark bg. Fix: switch to explicit text-slate-50/400,
  and :deep() override for form inputs via AuthLayout split scope.

Legacy (non-split) mode unaffected.

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

frontend/src/views/auth/LoginView.vue

@@ -21,8 +21,8 @@
     <div class="space-y-6">
       <!-- Title -->
       <div class="text-center">
-        <h2 class="text-2xl font-bold text-gray-900 dark:text-white">{{ t('auth.puroLoginTitle') }}</h2>
-        <p class="mt-2 text-sm text-gray-500 dark:text-dark-400">{{ t('auth.puroLoginSub') }}</p>
+        <h2 class="text-2xl font-bold text-slate-50">{{ t('auth.puroLoginTitle') }}</h2>
+        <p class="mt-2 text-sm text-slate-400">{{ t('auth.puroLoginSub') }}</p>
       </div>
 
   <div v-if="!backendModeEnabled && (linuxdoOAuthEnabled || oidcOAuthEnabled)" class="space-y-4">

frontend/src/views/auth/RegisterView.vue

@@ -21,8 +21,8 @@
     <div class="space-y-6">
       <!-- Title -->
       <div class="text-center">
-        <h2 class="text-2xl font-bold text-gray-900 dark:text-white">{{ t('auth.puroRegisterTitle') }}</h2>
-        <p class="mt-2 text-sm text-gray-500 dark:text-dark-400">{{ t('auth.puroRegisterSub') }}</p>
+        <h2 class="text-2xl font-bold text-slate-50">{{ t('auth.puroRegisterTitle') }}</h2>
+        <p class="mt-2 text-sm text-slate-400">{{ t('auth.puroRegisterSub') }}</p>
       </div>
 
       <div v-if="linuxdoOAuthEnabled || oidcOAuthEnabled" class="space-y-4">