feat(android): a shield instead of a padlock, and the launch screen matches

The padlock was drawn in thin blue strokes on slate: at 36 px the outline muddied and the shackle vanished. A filled white shield with a punched keyhole keeps its silhouette at every size, and says protection more plainly.

The keyhole is CUT OUT rather than painted, through an even-odd fill on a single path - painting it meant keeping two files agreeing on one colour, and any drift would have read as a smudge. The launch layer is the exception: it sits flat on the system background, where a cut-out would be a hole, so there the keyhole is painted and stays entirely inside the white shield.

The launch background takes the top of the app gradient instead of slate, so the launch screen dissolves into the first screen rather than cutting against it. No gradient inside the vectors: the aapt this chain uses does not know inline gradients.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

android-client/android-res/drawable/ic_launcher_monochrome.xml

@@ -1,22 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
-<!-- Version monochrome, pour les icones themees d'Android 13+. Le systeme
-     ignore les couleurs et ne garde que la silhouette, qu'il teinte selon le
-     fond d'ecran. La serrure doit donc etre un TROU dans le trace, pas une
-     forme d'une autre couleur - d'ou le remplissage evenOdd plutot que deux
-     chemins superposes. -->
+<!-- Couche monochrome (icones thematiques d'Android 13+). Le systeme ne
+     retient que la FORME : il repeint le dessin avec la couleur du theme de
+     l'utilisateur, donc la couleur posee ici n'a aucune importance.
+     Meme bouclier que l'avant-plan, serrure evidee de la meme facon. -->
 <vector xmlns:android="http://schemas.android.com/apk/res/android"
     android:width="108dp"
     android:height="108dp"
     android:viewportWidth="108"
     android:viewportHeight="108">
-    <path
-        android:strokeColor="#FFFFFF"
-        android:strokeWidth="5.5"
-        android:strokeLineCap="round"
-        android:fillColor="#00000000"
-        android:pathData="M44,56 L44,47 A10,10 0 0 1 64,47 L64,56" />
     <path
         android:fillColor="#FFFFFF"
         android:fillType="evenOdd"
-        android:pathData="M38,54 L70,54 A4,4 0 0 1 74,58 L74,76 A4,4 0 0 1 70,80 L38,80 A4,4 0 0 1 34,76 L34,58 A4,4 0 0 1 38,54 Z M54,62 m-4,0 a4,4 0 1,0 8,0 a4,4 0 1,0 -8,0 Z M52.2,66 L55.8,66 L55,73.5 L53,73.5 Z" />
+        android:pathData="M54,29 L77,37 L77,56 C77,69.5 67,78 54,82 C41,78 31,69.5 31,56 L31,37 Z
+                          M54,50 m-7,0 a7,7 0 1,0 14,0 a7,7 0 1,0 -14,0 Z
+                          M51,55.5 L57,55.5 L58.6,70 L49.4,70 Z" />
 </vector>