Show missing-credentials indicator for remote server sources

After reinstalling the app and importing sources from iCloud, remote
server instances with Keychain-only credentials showed no indication
that login is needed, unlike WebDAV/SMB sources.

- Show the orange key icon for Yattee Server instances without stored
  basic auth credentials (auth is always required for this type)
- Persist usesBasicAuth/usesAccountLogin flags on Instance (synced via
  iCloud) so missing Invidious/Piped/PeerTube credentials are detectable
  too; maintained on credential save/delete, login/logout, and legacy
  migration
- Centralize the check in AppEnvironment.needsCredentials(for:) used by
  SourcesListView, MediaSourcesView, and SourceRow
- Backfill the flags at launch from credentials currently in the
  Keychain so existing setups publish them without a re-login

Yattee/Views/Settings/AddSource/AddRemoteServerView.swift

@@ -753,7 +753,8 @@ struct AddRemoteServerView: View {
                             type: type,
                             url: url,
                             name: name.isEmpty ? nil : name,
-                            allowInvalidCertificates: allowInvalidCertificates
+                            allowInvalidCertificates: allowInvalidCertificates,
+                            usesBasicAuth: true
                         )
 
                         appEnvironment.basicAuthCredentialsManager.setCredentials(
@@ -783,14 +784,16 @@ struct AddRemoteServerView: View {
 
         // For other instance types: optionally store HTTP Basic Auth credentials
         // (used when the instance is fronted by a reverse proxy that requires basic auth).
+        let hasBasicAuth = !basicAuthUsername.isEmpty && !basicAuthPassword.isEmpty
         let instance = Instance(
             type: type,
             url: url,
             name: name.isEmpty ? nil : name,
-            allowInvalidCertificates: allowInvalidCertificates
+            allowInvalidCertificates: allowInvalidCertificates,
+            usesBasicAuth: hasBasicAuth
         )
 
-        if !basicAuthUsername.isEmpty, !basicAuthPassword.isEmpty {
+        if hasBasicAuth {
             appEnvironment.basicAuthCredentialsManager.setCredentials(
                 username: basicAuthUsername,
                 password: basicAuthPassword,