feat: default login-screen wallpaper

Ship a default wallpaper (blossoms by Nikhil Kumar, Unsplash License) in
a new 0:/os/wallpapers/ directory for OS-provided imagery, staged from
programs/data/wallpapers/. login.elf falls back to
0:/os/wallpapers/default.jpg when the desktop config names no wallpaper
or the configured file cannot be opened. Attribution added to
THIRD-PARTY-NOTICES.txt and man legal.

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

programs/man/legal.7

@@ -53,5 +53,7 @@
       Program - MPL-2.0
     * Intel Bluetooth firmware (0:/os/firmware/intel), Copyright (c) Intel
       Corporation - Intel redistributable firmware license
+    * Default wallpaper photo (0:/os/wallpapers), by Nikhil Kumar -
+      Unsplash License
 
     Full license texts and notices are on this system in 0:/os/licenses/.