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/src/login/login_wallpaper.cpp

@@ -8,6 +8,9 @@
 
 namespace {
 
+// Shipped fallback shown when no wallpaper is configured (see NOTICES.txt).
+constexpr const char* kDefaultWallpaperPath = "0:/os/wallpapers/default.jpg";
+
 constexpr uint32_t kLoginOverlayAlpha = 0x38;
 constexpr uint32_t kLoginOverlayInvAlpha = 255 - kLoginOverlayAlpha;
 
@@ -23,9 +26,10 @@ bool load_login_wallpaper(LoginState* ls) {
     char wp[256];
     montauk::strncpy(wp, doc.get_string("wallpaper.path", ""), sizeof(wp));
     doc.destroy();
-    if (wp[0] == '\0') return false;
 
-    int fd = montauk::open(wp);
+    int fd = -1;
+    if (wp[0] != '\0') fd = montauk::open(wp);
+    if (fd < 0) fd = montauk::open(kDefaultWallpaperPath);
     if (fd < 0) return false;
 
     uint64_t size = montauk::getsize(fd);