feat(studio): contextual "Add registry token" CTA on install auth wall (v0.46.0)

The hub's v0.10.91 install error message already tells the
operator exactly what to do — set FAI_REGISTRY_TOKEN or save
to ~/.fai/registry-token via Settings. But that text lives in
a FaiErrorBox inside the install dialog. Reading the buried
recovery instruction, finding Settings, scrolling to the
Registry credentials panel, pasting, then re-clicking the
install pill is five hops for what should be one.

When at least one item in the install dialog fails with the
"registry returned an HTML page" / "no registry token
configured" signature, the dialog now surfaces a prominent
filled "Add registry token" button next to Close. Click closes
the install dialog and opens the Settings dialog directly so
the operator pastes + saves without losing context. After
Settings dismisses, the operator is back at the flow card and
re-clicks the same pill to retry.

Detection is signature-based (substring match on the hub's
error text) so it activates whether the install was a single-
pill click or an "Install all" sequence. Other failure modes
(network down, signature invalid, sha mismatch) still show
just the Close button — the CTA is specific to the auth-wall
case it can actually unblock.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/pages/flows.dart

@@ -940,6 +940,22 @@ class _InstallDependenciesDialogState
   int get _failedCount =>
       _items.where((i) => i.status == _InstallStatus.failed).length;
 
+  /// True iff at least one failure carries the v0.10.91+ HTML-instead-
+  /// of-bundle signature emitted by `download_to_temp` when the
+  /// registry redirects an unauthenticated request to a signin page.
+  /// Used to surface a contextual "Open Settings" button so the
+  /// operator does not have to read the wall of error text first.
+  bool get _anyAuthWallFailure => _items.any(
+        (i) =>
+            i.status == _InstallStatus.failed && _isAuthWallError(i.error),
+      );
+
+  static bool _isAuthWallError(String? msg) {
+    if (msg == null) return false;
+    return msg.contains('registry returned an HTML page') ||
+        msg.contains('no registry token configured');
+  }
+
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
@@ -986,6 +1002,18 @@ class _InstallDependenciesDialogState
         ),
       ),
       actions: [
+        if (!_running && _anyAuthWallFailure)
+          FilledButton.icon(
+            icon: const Icon(Icons.vpn_key_outlined, size: 16),
+            label: Text(l.flowsInstallAuthWallButton),
+            onPressed: () async {
+              // Close install dialog first so the operator returns
+              // to the flow card on Settings dismiss — then they
+              // can re-click the pill to retry the install.
+              Navigator.pop(context, _anySucceeded);
+              await FaiSettingsDialog.show(context);
+            },
+          ),
         TextButton(
           onPressed: _running
               ? null