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.

pubspec.yaml

@@ -75,4 +75,26 @@ flutter:
     - assets/audio/
     - assets/mockups/
     - assets/generated/
+    # Flutter NO recurse: declarar 'assets/content/' incluye solo los
+    # ficheros sueltos de esa carpeta, nunca los de sus subcarpetas. Todo
+    # el contenido vive en subcarpetas, asi que NADA de esto viajaba en el
+    # APK -- verificado abriendo el binario instalado: cero entradas de
+    # assets/content. El onboarding reventaba en cada arranque con
+    # 'Unable to load asset: assets/content/onboarding/en.md' aunque el
+    # fichero existe en disco. Mismo fallo de familia que los drawables
+    # resueltos por nombre: referencia sin validacion en compilacion.
     - assets/content/
+    - assets/content/onboarding/
+    - assets/content/updates/ar/
+    - assets/content/updates/bn/
+    - assets/content/updates/de/
+    - assets/content/updates/en/
+    - assets/content/updates/es/
+    - assets/content/updates/fr/
+    - assets/content/updates/hi/
+    - assets/content/updates/id/
+    - assets/content/updates/it/
+    - assets/content/updates/ja/
+    - assets/content/updates/pt/
+    - assets/content/updates/ru/
+    - assets/content/updates/zh/