refactor(tenants): clean up Visibility tab copy after global-appearance removal

Updates the tenant edit Visibility tab so its labels and helper text
reflect the new tenant-only appearance model. After the global appearance
settings were removed, legends like "Use default color" / "Inherit global
setting" and help texts like "applies the global system appearance" /
"uses its own default theme instead of the global setting" were wrong —
there is no global appearance to inherit from anymore. An empty tenant
field means "use the system default" (hardcoded light theme, no brand
accent, user-theme allowed).

- Info blockquote at the top rewritten.
- "Use default color" → "No brand color" / "Use system default (no brand
  accent)" with updated helper text.
- Default theme placeholder "Inherit global setting" → "Use system default
  (light)" + new helper text.
- User-theme policy "Inherit global setting" → "Use system default
  (allowed)".

i18n: removed the now-orphaned keys ("Use default color", "Using the
default color applies the global system appearance.", "Be careful - this
resets the tenants color", "Inherit global setting", "If set, this tenant
uses its own default theme instead of the global setting."). Added the
new keys in de + en.

No behavior change — only copy. PHPStan, PHPUnit green.

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

pages/admin/tenants/_form.phtml

@@ -933,7 +933,7 @@ $openOverrideCard = $detailsOpenAll;
 
     <div data-tab-panel="visibility">
       <blockquote data-variant="info">
-        <?php e(t('Using the default color applies the global system appearance.')); ?>
+        <?php e(t('Appearance is controlled per tenant. If a field is left at the system default, the app renders in light theme without a brand accent.')); ?>
       </blockquote>
       <div class="grid">
         <fieldset>
@@ -951,7 +951,7 @@ $openOverrideCard = $detailsOpenAll;
         <fieldset>
           <legend>
             <small>
-              <?php e(t('Use default color')); ?>
+              <?php e(t('No brand color')); ?>
             </small>
           </legend>
           <label class="app-field">
@@ -962,29 +962,29 @@ $openOverrideCard = $detailsOpenAll;
               data-color-default="<?php e($defaultPrimaryColor); ?>"
               <?php e($useDefaultPrimaryColor ? 'checked' : ''); ?>
               <?php e($readonlyAttr); ?>>
-            <span><?php e(t('Use default color')); ?></span>
+            <span><?php e(t('Use system default (no brand accent)')); ?></span>
           </label>
-          <small class="muted"><?php e(t('Be careful - this resets the tenants color')); ?></small>
+          <small class="muted"><?php e(t('When enabled the tenant renders without a brand accent color.')); ?></small>
         </fieldset>
       </div>
       <div class="grid">
         <fieldset>
           <legend><small><?php e(t('Default theme')); ?></small></legend>
           <select name="default_theme" <?php e($disabledAttr); ?>>
-            <option value=""><?php e(t('Inherit global setting')); ?></option>
+            <option value=""><?php e(t('Use system default (light)')); ?></option>
             <?php foreach ($themes as $themeKey => $themeLabel): ?>
               <option value="<?php e($themeKey); ?>" <?php e($tenantDefaultTheme === $themeKey ? 'selected' : ''); ?>>
                 <?php e(t($themeLabel)); ?>
               </option>
             <?php endforeach; ?>
           </select>
-          <small class="muted"><?php e(t('If set, this tenant uses its own default theme instead of the global setting.')); ?></small>
+          <small class="muted"><?php e(t('The theme used for users in this tenant who have no personal preference.')); ?></small>
         </fieldset>
         <fieldset>
           <legend><small><?php e(t('User theme policy')); ?></small></legend>
           <select name="allow_user_theme_mode" <?php e($disabledAttr); ?>>
             <option value="" <?php e($tenantAllowUserThemeMode === '' ? 'selected' : ''); ?>>
-              <?php e(t('Inherit global setting')); ?>
+              <?php e(t('Use system default (allowed)')); ?>
             </option>
             <option value="1" <?php e($tenantAllowUserThemeMode === '1' ? 'selected' : ''); ?>>
               <?php e(t('Force allow user theme')); ?>