feat(studio): registry credentials panel in Settings (v0.44.0)

The hub install path now reads `~/.fai/registry-token` as a
fallback when `FAI_REGISTRY_TOKEN` is unset (platform v0.10.92).
Studio now writes that file directly: a fresh operator pastes
the PAT into Settings → Registry credentials, hits Save, and
the next install attempt resolves the auth wall without any
shell or env-var setup.

The token never round-trips back into Studio after save —
status is shown only as "Configured (40 chars)" / "Not set"
with no display of the secret itself. The Clear action deletes
the file. On Unix the file is chmod-ed to 0600 (owner-only
read/write); Windows leaves the default user ACL in place.

Storage is strictly local: `~/.fai/registry-token`, never sent
to a remote service. The hint text under the field says so to
make the data flow explicit.

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

lib/data/registry_token.dart

@@ -0,0 +1,66 @@
+import 'dart:io';
+import 'package:path/path.dart' as p;
+
+/// Operator-managed registry auth token, kept at
+/// `~/.fai/registry-token` (mode 0600 on Unix). The hub reads
+/// this file at install time when `FAI_REGISTRY_TOKEN` is unset
+/// — see `download_to_temp` in `crates/fai_hub/src/lib.rs`.
+///
+/// Studio writes the file directly so a fresh install never
+/// requires the operator to fiddle with shell env vars.
+class RegistryToken {
+  static String _faiHome() {
+    final home = Platform.environment['HOME'] ?? Platform.environment['USERPROFILE'];
+    if (home == null || home.isEmpty) {
+      throw StateError('Cannot resolve home directory (no HOME / USERPROFILE)');
+    }
+    return p.join(home, '.fai');
+  }
+
+  /// Absolute path to the token file.
+  static String get path => p.join(_faiHome(), 'registry-token');
+
+  /// True iff the file exists with a non-empty trimmed body.
+  static Future<bool> isConfigured() async {
+    final f = File(path);
+    if (!await f.exists()) return false;
+    final content = await f.readAsString();
+    return content.trim().isNotEmpty;
+  }
+
+  /// Length of the trimmed token, or null when the file is
+  /// missing / empty. Used for status display ("Configured
+  /// (40 chars)") without ever surfacing the secret.
+  static Future<int?> charCount() async {
+    final f = File(path);
+    if (!await f.exists()) return null;
+    final content = await f.readAsString();
+    final trimmed = content.trim();
+    return trimmed.isEmpty ? null : trimmed.length;
+  }
+
+  /// Persist [token] to disk, creating `~/.fai/` if needed.
+  /// On Unix the file is chmod-ed to 0600 (owner read/write
+  /// only). On Windows the default user-ACL is left alone —
+  /// best-effort, no PowerShell handshake.
+  static Future<void> write(String token) async {
+    final f = File(path);
+    await f.parent.create(recursive: true);
+    await f.writeAsString(token.trim(), flush: true);
+    if (Platform.isLinux || Platform.isMacOS) {
+      try {
+        await Process.run('chmod', ['600', f.path]);
+      } catch (_) {
+        // best-effort; if chmod is unavailable the file still
+        // exists with default perms (usually 0644, world-readable
+        // but only on the operator's own machine).
+      }
+    }
+  }
+
+  /// Delete the token file. No-op when the file doesn't exist.
+  static Future<void> delete() async {
+    final f = File(path);
+    if (await f.exists()) await f.delete();
+  }
+}