feat: add a hub for the client's own pages and gate mews:do action links

app/lib/application/internal_actions.dart

@@ -0,0 +1,36 @@
+import 'controllers/reader_settings_controller.dart'
+    show ReaderSettingsController;
+import 'pages/mews_urls.dart';
+
+/// Applies the change an action address encodes, e.g. mews:do/set-theme.
+/// Returns whether anything changed, so the navigator can skip a rerender that
+/// would show the same page.
+///
+/// An unknown verb or a missing argument does nothing and reports nothing: the
+/// only links carrying these addresses are the ones the client builds, so a
+/// miss is a bug rather than something to explain to the reader.
+class InternalActions {
+  InternalActions({required ReaderSettingsController settings})
+      : _settings = settings;
+
+  final ReaderSettingsController _settings;
+
+  Future<bool> apply(String url) async {
+    final verb = MewsUrls.verbOf(url);
+    if (verb == null) return false;
+    final args = Uri.parse(url).queryParameters;
+    return switch (verb) {
+      'set-font' => _settings.setFont(args['value'] ?? ''),
+      'set-size' => _setSize(args['step']),
+      'set-theme' => _settings.setThemeMode(args['value'] ?? ''),
+      'set-colors' => _settings.setVariant(args['value'] ?? ''),
+      _ => false,
+    };
+  }
+
+  Future<bool> _setSize(String? step) => switch (step) {
+    'up' => _settings.increaseSize(),
+    'down' => _settings.decreaseSize(),
+    _ => Future.value(false),
+  };
+}