Feature: admin toggle for search-engine indexing (robots)

Add a 'Search Engine Indexing' toggle under Settings -> General. Off (the
default) emits robots noindex/nofollow in the document head - the safe default
for a private webmail; on lets an admin opt the deployment into indexing.
Backed by the existing admin config-manager (SEARCH_ENGINE_INDEXING env var /
admin override / revert), read server-side in the root generateMetadata().

app/(main)/admin/_tabs/settings.tsx

@@ -125,6 +125,7 @@ export function SettingsTab() {
         )}
         <ToggleSetting label="Stalwart Features" description="Enable Stalwart Mail Server-specific features" configKey="stalwartFeaturesEnabled" value={currentValue('stalwartFeaturesEnabled') as boolean} source={config.stalwartFeaturesEnabled?.source} onChange={handleChange} onRevert={handleRevert} />
         <ToggleSetting label="Demo Mode" description="Enable demo mode with sample data" configKey="demoMode" value={currentValue('demoMode') as boolean} source={config.demoMode?.source} onChange={handleChange} onRevert={handleRevert} />
+        <ToggleSetting label="Search Engine Indexing" description="Allow search engines to index this webmail. Off (the default) sends noindex/nofollow in the page head, recommended for private deployments." configKey="searchEngineIndexing" value={currentValue('searchEngineIndexing') as boolean} source={config.searchEngineIndexing?.source} onChange={handleChange} onRevert={handleRevert} />
       </SettingsSection>
 
       <SettingsSection title="JMAP Servers (multi-server)">