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>

montaukos.org/THIRD-PARTY-NOTICES.txt

@@ -355,6 +355,25 @@ License: Intel redistributable firmware license
   the MontaukOS ISO at 0:/os/licenses/Intel-ibt-firmware.txt.
 
 
+================================================================================
+14. Default wallpaper photograph
+================================================================================
+Shipped as the default login-screen wallpaper on the ISO
+(0:/os/wallpapers/default.jpg): a photograph of blossoms against misty hills.
+
+  Photo by Nikhil Kumar on Unsplash
+  Source: https://unsplash.com/photos/JdPHvI7VF0o
+
+License: Unsplash License (https://unsplash.com/license)
+
+  Unsplash grants you an irrevocable, nonexclusive, worldwide copyright
+  license to download, copy, modify, distribute, perform, and use images
+  from Unsplash for free, including for commercial purposes, without
+  permission from or attributing the photographer or Unsplash. This license
+  does not include the right to compile images from Unsplash to replicate a
+  similar or competing service.
+
+
 ================================================================================
 For licensing inquiries regarding MontaukOS itself, contact: daniel@nexlink.cloud
 ================================================================================

programs/GNUmakefile

@@ -94,9 +94,14 @@ LICDIR := data/licenses
 LICSRC := $(wildcard $(LICDIR)/*.txt)
 LICDST := $(patsubst $(LICDIR)/%,$(BINDIR)/os/licenses/%,$(LICSRC)) $(BINDIR)/os/licenses/NOTICES.txt $(BINDIR)/os/licenses/LICENSE.txt
 
+# Wallpapers bundled into bin/os/wallpapers/.
+WPDIR := data/wallpapers
+WPSRC := $(wildcard $(WPDIR)/*.jpg)
+WPDST := $(patsubst $(WPDIR)/%,$(BINDIR)/os/wallpapers/%,$(WPSRC))
+
 .PHONY: all clean 2048 doom fetch wiki wikipedia weather imageviewer fontpreview spreadsheet wordprocessor pdfviewer disks devexplorer installer audio music video bluetooth network terminal klog procmgr powermgr calculator charmap login desktop shell rpgdemo paint tcc lua screenshot texteditor mandelbrot printers timezone printd printctl dialogs icons fonts configs bearssl libc tls libjpeg libjpegwrite install-apps libloader libs crashpad check-syscalls gen-syscalls
 
-all: bearssl libc libjpeg libjpegwrite tls libloader libs $(TARGETS) fetch wiki wikipedia weather imageviewer fontpreview spreadsheet wordprocessor pdfviewer disks devexplorer installer audio music video bluetooth network terminal klog procmgr powermgr calculator charmap 2048 doom rpgdemo paint tcc lua screenshot texteditor mandelbrot printers timezone printd printctl dialogs login desktop shell icons fonts install-apps crashpad $(MANDST) $(WWWDST) $(CA_CERTS) $(CONFIGDST) $(FWDST) $(LICDST)
+all: bearssl libc libjpeg libjpegwrite tls libloader libs $(TARGETS) fetch wiki wikipedia weather imageviewer fontpreview spreadsheet wordprocessor pdfviewer disks devexplorer installer audio music video bluetooth network terminal klog procmgr powermgr calculator charmap 2048 doom rpgdemo paint tcc lua screenshot texteditor mandelbrot printers timezone printd printctl dialogs login desktop shell icons fonts install-apps crashpad $(MANDST) $(WWWDST) $(CA_CERTS) $(CONFIGDST) $(FWDST) $(LICDST) $(WPDST)
 
 # Build BearSSL static library (cross-compiled for freestanding x86_64).
 BEARSSL_INCLUDES := -isystem $(shell cd .. && pwd)/kernel/freestnd-c-hdrs/x86_64/include -isystem $(abspath include/libc)
@@ -370,6 +375,11 @@ $(BINDIR)/os/licenses/%: $(LICDIR)/%
 	mkdir -p $(BINDIR)/os/licenses
 	cp $< $@
 
+# Copy wallpapers into bin/os/wallpapers/.
+$(BINDIR)/os/wallpapers/%: $(WPDIR)/%
+	mkdir -p $(BINDIR)/os/wallpapers
+	cp $< $@
+
 # Build each system program from its source files.
 $(BINDIR)/os/%.elf: src/%/main.cpp link.ld GNUmakefile
 	mkdir -p $(BINDIR)/os obj/$*

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/.

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);