fix(assets): declare the content subdirectories so onboarding ships

Audit of the same failure family as the shrunk drawables: references by
NAME that nothing validates at compile time.

The whole onboarding and release-notes feature had never shipped. Reading
the installed APK: ZERO entries under assets/content/, while
assets/icons/alarmas/* was present. pubspec declared `assets/content/`, and
Flutter does not recurse -- naming a directory includes the files sitting
directly in it, never its subdirectories. Every content file lives in one
(onboarding/, updates/<locale>/), so none of them were packaged.

On the device that surfaced on every single launch:

  Unable to load asset: "assets/content/onboarding/en.md"

with the file plainly present on disk. That is why it never looked like a
packaging problem. The tell was already in the pubspec: assets/icons/alarmas/
is listed explicitly, so the rule was known once and not applied here.

All 14 content directories are now declared: onboarding/ plus updates/ for
each of the 13 locales.

The guard is a test that loads every file under assets/content/ through
rootBundle, because that is the only thing that proves an asset is declared
and will ship. A test asserting File.existsSync would have stayed green
through all of this -- the files were never missing, only unpackaged. Run
against the unfixed pubspec it fails 26 of 27; with the fix it passes.

Tests: 1165 -> 1192.

test/assets_contenido_declarados_test.dart

@@ -0,0 +1,55 @@
+import 'dart:io';
+
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+/// Every file under `assets/content/` must be loadable through `rootBundle`,
+/// which is the only thing that proves it is DECLARED in pubspec.yaml and
+/// therefore actually ships.
+///
+/// Found by reading the installed APK: it contained ZERO entries under
+/// `assets/content/`, while `assets/icons/alarmas/*` was present. pubspec
+/// declared `assets/content/` — and Flutter does NOT recurse: naming a
+/// directory includes the files sitting directly in it, never its
+/// subdirectories. All of this content lives in subdirectories
+/// (`onboarding/`, `updates/<locale>/`), so the entire onboarding and
+/// release-notes feature had never shipped in any build. On the device it
+/// surfaced on every launch as:
+///
+///   Unable to load asset: "assets/content/onboarding/en.md"
+///
+/// with the file plainly present on disk.
+///
+/// Same family as the drawables the resource shrinker deleted: a reference by
+/// NAME that nothing validates at compile time, so it fails only on a device.
+/// A test that merely checked `File(...).existsSync()` would have stayed green
+/// throughout — the files were never missing. Loading through `rootBundle` is
+/// what makes it a real guard, because that is the path the app itself takes.
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  final directorio = Directory('assets/content');
+  final ficheros =
+      directorio
+          .listSync(recursive: true)
+          .whereType<File>()
+          .map((f) => f.path.replaceAll(r'\', '/'))
+          .toList()
+        ..sort();
+
+  test('hay contenido que comprobar (si no, este test sería vacuo)', () {
+    expect(ficheros, isNotEmpty);
+  });
+
+  for (final ruta in ficheros) {
+    test('$ruta está declarado y se puede cargar', () async {
+      await expectLater(
+        rootBundle.loadString(ruta),
+        completes,
+        reason:
+            'existe en disco pero rootBundle no lo encuentra: falta declarar '
+            'su directorio en pubspec.yaml, y no viajará en el APK',
+      );
+    });
+  }
+}