feat(gui,kernel): 1080p HD desktop, polished chrome, ktime clock + live FPS

GUI polish / HD:
- Default display mode raised to 1920x1080 (video=1920x1080); make run/vnc use
  vgamem_mb=64 and -m 512M. The growable heap absorbs the larger backbuffer,
  shadow and wallpaper (observed: one 16 MB growth to a 48 MB heap).
- Wallpaper: three-stop vertical gradient, a soft concentric-frame edge vignette
  for depth, a finer faint grid, and a centred brand mark.
- Window chrome: brighter/saturated titlebar gradient when focused, an accent
  underline on the active window, and larger/softer drop shadows.
- Default session windows are placed and sized for the HD work area.
- Live FPS pill in the taskbar tray (green/amber/red by rate); tray widened so
  FPS + CPU trace + memory badge + clock no longer overlap.

Kernel:
- kernel/time/ktime.c + include/kernel/ktime.h (new): a unified monotonic
  high-resolution clock that prefers the HPET (nanosecond) and falls back to the
  100 Hz PIT tick. Wired ktime_init() into boot; drives the compositor's FPS
  sampling. New `ktime` LazyBox applet reports the source and uptime.

Verified live (screenshotted): boots to 1920x1080, KTIME reports
"source: HPET, nanosecond resolution", the polished desktop renders with the
active-window accent and vignette, and the tray shows a live FPS reading.
Clean build on x86_64/aarch64/armv8i; 0 panics / 0 OOM.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Fh5z6V7nGPbHvqVG5p4CXX

include/kernel/ktime.h

@@ -0,0 +1,17 @@
+#ifndef _KERNEL_KTIME_H
+#define _KERNEL_KTIME_H
+
+// Unified monotonic high-resolution clock. Prefers the HPET (nanosecond grade)
+// and falls back to the 100 Hz timer tick when no HPET is present.
+
+#include <stdint.h>
+#include <stdbool.h>
+
+void        ktime_init(void);
+uint64_t    ktime_ns(void);          // nanoseconds since ktime_init
+uint64_t    ktime_us(void);
+uint64_t    ktime_ms(void);
+const char* ktime_source(void);      // "HPET" or "PIT"
+bool        ktime_is_highres(void);  // true when backed by the HPET
+
+#endif // _KERNEL_KTIME_H