feat(gui): Implement complete Graphical Desktop Environment, SUB-WM, and apps

- Create separate feature branch 'feat/gui-desktop'
- Implement double-buffered 2D graphics engine with alpha blending, gradients, rounded rects, and font rasterization in gui/gui_gfx.c and include/gui/gui_gfx.h
- Implement alpha-blended mouse pointer overlay in gui/gui_cursor.c and include/gui/gui_cursor.h
- Implement SUB-WM Window Manager with draggable titlebars, close/minimize buttons, drop shadows, and event routing in gui/gui_wm.c and include/gui/gui_wm.h
- Implement Cyberpunk/Nord desktop environment with wallpaper, taskbar, start menu popup, and system tray in gui/gui_desktop.c and include/gui/gui_desktop.h
- Implement built-in GUI apps: System Performance Monitor, GUI Terminal Emulator, File Explorer, Calculator, Paint Studio, and About dialog in gui/gui_apps.c and include/gui/gui_apps.h
- Add 'startx', 'gui', and 'desktop' applets in LazyBox suite
- Verified multi-architecture compilation across x86_64, AArch64, and ARMv8i

include/gui/gui_gfx.h

@@ -0,0 +1,40 @@
+#ifndef _GUI_GFX_H
+#define _GUI_GFX_H
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <gui/gui_theme.h>
+
+typedef struct {
+    int x;
+    int y;
+    int width;
+    int height;
+} gui_rect_t;
+
+void gui_gfx_init(uint32_t* fb, int width, int height);
+int gui_gfx_get_width(void);
+int gui_gfx_get_height(void);
+uint32_t* gui_gfx_get_backbuffer(void);
+
+void gui_gfx_clear(uint32_t color);
+void gui_gfx_draw_pixel(int x, int y, uint32_t color);
+void gui_gfx_draw_pixel_blend(int x, int y, uint32_t color, uint8_t alpha);
+void gui_gfx_draw_line(int x0, int y0, int x1, int y1, uint32_t color);
+void gui_gfx_draw_rect(int x, int y, int w, int h, uint32_t color);
+void gui_gfx_fill_rect(int x, int y, int w, int h, uint32_t color);
+void gui_gfx_fill_rect_blend(int x, int y, int w, int h, uint32_t color, uint8_t alpha);
+void gui_gfx_draw_rounded_rect(int x, int y, int w, int h, int r, uint32_t color, bool filled);
+void gui_gfx_draw_gradient_v(int x, int y, int w, int h, uint32_t top_col, uint32_t bot_col);
+void gui_gfx_draw_gradient_h(int x, int y, int w, int h, uint32_t left_col, uint32_t right_col);
+
+void gui_gfx_draw_char(int x, int y, char c, uint32_t fg, uint32_t bg, bool transparent_bg);
+void gui_gfx_draw_string(int x, int y, const char* str, uint32_t fg);
+void gui_gfx_draw_string_bg(int x, int y, const char* str, uint32_t fg, uint32_t bg);
+void gui_gfx_draw_string_shadow(int x, int y, const char* str, uint32_t fg, uint32_t shadow);
+void gui_gfx_draw_shadow(int x, int y, int w, int h, int blur);
+void gui_gfx_blit(int dx, int dy, int w, int h, const uint32_t* src_buf);
+void gui_gfx_present(void);
+
+#endif // _GUI_GFX_H