feat: add Gemini and Spartan protocol support

app/lib/application/key_store.dart

@@ -0,0 +1,41 @@
+import 'dart:io';
+
+/// The private keys of imported client certificates, as PEM files in the app's
+/// own storage. The database keeps the public certificate; the key stays out
+/// of it, so a copy of the database never carries a reader's identity.
+///
+/// The operations are synchronous on purpose: they touch one small file at a
+/// time, only when the reader imports or uses an identity, and staying off the
+/// event loop keeps them completing within a single interaction.
+class KeyStore {
+  KeyStore(this._root);
+
+  /// The directory the keys live in, resolved lazily because finding it asks
+  /// the platform.
+  final Future<Directory> Function() _root;
+
+  Future<File> _file(int id) async {
+    final root = await _root();
+    return File('${root.path}${Platform.pathSeparator}$id.pem');
+  }
+
+  /// Stores the key of the certificate with this id.
+  Future<void> save(int id, String pem) async {
+    final file = await _file(id);
+    file.parent.createSync(recursive: true);
+    file.writeAsStringSync(pem);
+  }
+
+  /// The key of the certificate with this id, or null when there is none.
+  Future<String?> read(int id) async {
+    final file = await _file(id);
+    if (!file.existsSync()) return null;
+    return file.readAsStringSync();
+  }
+
+  /// Removes the key of the certificate with this id.
+  Future<void> delete(int id) async {
+    final file = await _file(id);
+    if (file.existsSync()) file.deleteSync();
+  }
+}