fix(http): block /install page after setup is complete

- skip SPA fallback for `/install` and `/install/*` in main server
- keep install wizard accessible only during `RunInstallServer` phase
- prevent post-install access to install UI

src/model/data/settings_data.go

@@ -151,6 +151,10 @@ func DeleteSetting(key string) error {
 // sensitiveSettingKeys lists keys that must never be returned to API callers.
 var sensitiveSettingKeys = []string{
 	mdb.SettingKeyJwtSecret,
+	mdb.SettingKeyInitAdminPasswordPlain,
+	mdb.SettingKeyInitAdminPasswordHash,
+	mdb.SettingKeyInitAdminPasswordFetched,
+	mdb.SettingKeyInitAdminPasswordChanged,
 }
 
 // ListSettingsByGroup returns all rows for a given group (empty group = all),