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/SourceRow.swift

@@ -32,6 +32,13 @@ struct SourceRow: View {
         return status == .authFailed || status == .authRequired
     }
 
+    /// Whether a remote server instance requires credentials that are missing from
+    /// the Keychain (e.g. after reinstalling and importing sources from iCloud).
+    private var needsCredentials: Bool {
+        guard case .remoteServer(let instance) = source else { return false }
+        return appEnvironment?.needsCredentials(for: instance) ?? false
+    }
+
     var body: some View {
         #if os(tvOS)
         Button(action: onEdit) {
@@ -92,7 +99,7 @@ struct SourceRow: View {
 
     @ViewBuilder
     private var statusView: some View {
-        if needsPassword {
+        if needsPassword || needsCredentials {
             Label(String(localized: "sources.status.authRequired"), systemImage: "key.fill")
                 .font(.caption2)
                 .foregroundStyle(.orange)