refactor(settings): remove global appearance settings — tenant is sole source

Removes the global app_theme, app_theme_user and app_primary_color settings
from the admin/settings area and the underlying service/cache/API/i18n/docs
layers. The tenant columns (tenants.primary_color, default_theme,
allow_user_theme) become the single source of truth for appearance.
Branding helpers are tenant-only and fall back to a hardcoded 'light' theme
with no brand color when no tenant context is available (login, error,
pre-session pages). The APP_THEME env var is removed.

Scope:
- UI: Appearance tab gone from /admin/settings; tenant edit form drops the
  global-fallback color preview.
- Service: SettingsAppGateway no longer exposes setting-backed accessors
  for theme/user-theme/primary-color. Theme catalog utilities (listThemes,
  isAllowedTheme, normalizeTheme, resolveDefaultTheme) stay and are used
  across Tenant / Auth / User flows; resolveDefaultTheme now hardcodes
  'light' with a catalog fallback (no global/env lookup).
- AdminSettingsService: buildPageData, sanitization, audit snapshot, apply
  step and cache payload are all stripped of the three keys. Dead helper
  applyAppPrimaryColor + normalizePrimaryColor removed.
- Cache: SettingCacheService HOT_PATH_KEYS drops the three keys.
- API: /settings (GET/PUT) and /settings/public (GET) no longer expose
  appearance fields; OpenAPI schemas pruned accordingly.
- Audit redaction list shortened.
- DB: db/init/init.sql seed rows removed. No migration for existing
  installs — legacy rows stay inert.
- i18n + docs: setting.app_theme, setting.app_theme_user,
  setting.app_primary_color removed from de+en locales; reference and
  explanation docs updated.
- Tests: covering tests for the removed methods pruned; ThemeResolutionTest
  rewritten for tenant-only semantics. One unrelated PHP-CS-Fixer issue in
  UserRegistrar.php cleaned up to keep QG-006 green.

Workflow: .agents/runs/SETTINGS-APPEARANCE-TENANT-ONLY/ (gitignored).
Gates: QG-001..003, QG-006, QG-008 all pass (1985 tests, 28764 assertions).
Reviews: code, security, acceptance all pass.

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

tests/Service/Settings/AdminSettingsServiceSecurityTest.php

@@ -396,8 +396,6 @@ class AdminSettingsServiceSecurityTest extends TestCase
             'default_department_id' => '0',
             'app_title' => 'Demo',
             'app_locale' => 'de',
-            'app_theme' => 'light',
-            'app_theme_user' => '1',
             'app_registration' => '1',
             'api_token_default_ttl_days' => '90',
             'api_token_max_ttl_days' => '365',
@@ -411,7 +409,6 @@ class AdminSettingsServiceSecurityTest extends TestCase
             'frontend_telemetry_enabled' => '1',
             'frontend_telemetry_sample_rate' => '0.2',
             'frontend_telemetry_allowed_events' => ['warn_once'],
-            'app_primary_color' => '#2FA4A4',
             'smtp_host' => '',
             'smtp_port' => '',
             'smtp_user' => '',
@@ -443,16 +440,10 @@ class AdminSettingsServiceSecurityTest extends TestCase
 
         $settingsAppGateway = $this->createConfiguredMock(SettingsAppGateway::class, [
             'getAppLocale' => 'de',
-            'getAppTheme' => 'light',
-            'isUserThemeAllowed' => true,
             'isRegistrationEnabled' => true,
-            'getAppPrimaryColor' => '#2FA4A4',
             'setAppTitle' => true,
             'setAppLocale' => true,
-            'setAppTheme' => true,
-            'setUserThemeAllowed' => true,
             'setRegistrationEnabled' => true,
-            'setAppPrimaryColor' => true,
         ]);
 
         $settingsApiPolicyGateway = $this->createConfiguredMock(SettingsApiPolicyGateway::class, [