feat: Growable heap, GUI-by-default run, live sysmon, SUB/SUB default login

Kernel / memory:
- mm/kmalloc.c: fix "Heap: Out of memory" under the GUI. The desktop allocates
  three full-screen 32-bit buffers (~11MB) which overflowed the fixed 16MB pool.
  The heap now starts at 32MB and grows on demand from the PMM (16MB regions),
  with region-safe coalescing (only merges physically adjacent blocks) so growth
  never corrupts across separate PMM allocations. Adds heap_get_grow_count().

Build / run:
- Makefile: merge `run-gui` into `run` so `make run` boots into the graphical
  desktop; add `run-headless` (serial-only via QEMU_CMD), bump guest RAM to
  256M, update .PHONY and help.

Configure:
- scripts/kconfig/menuconfig.py: gate the curses UI behind isatty() on stdin and
  stdout so non-interactive `make configure` stops emitting "nocbreak() returned
  ERR" and falls cleanly through to the non-interactive writer.

GUI / userland:
- gui/gui_apps.c: rebuild the System Monitor from hardcoded values into live
  telemetry -- real heap used/total/free with a colour-shifting bar, a scrolling
  heap-utilisation graph, the grow counter, and real PMM RAM figures.
- lazybox `free`: report how many times the heap has grown.

Auth:
- Default login is now SUB / SUB (Kali-style username==password), seeded first
  as the primary uid-0 account and set as the logged-in user; root kept for
  compatibility. Hardcoded "root" defaults (whoami, su, passwd, ssh, sshd
  session) updated to SUB.

Verified: clean build on x86_64/aarch64/armv8i; GUI boots with 0 OOM/0 panic;
auth engine initialises cleanly; `make configure` fallback is clean.

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

security/auth.c

@@ -23,11 +23,17 @@ void auth_init(void) {
     user_count = 0;
     current_uid = 0;
 
-    // Seed default users
-    auth_add_user("root", "root", 0, 0, "/root", "/bin/sh");
+    // Seed default users. SUB/SUB is the primary superuser account -- the way
+    // Kali ships with kali/kali -- so the username and password both default to
+    // "SUB". It is seeded first, so uid 0 resolves to it as the logged-in user.
+    auth_add_user("SUB", "SUB", 0, 0, "/root", "/bin/sh");
+    auth_add_user("root", "root", 0, 0, "/root", "/bin/sh"); // kept for compatibility
     auth_add_user("admin", "admin123", 1000, 1000, "/home/admin", "/bin/sh");
     auth_add_user("guest", "guest", 1001, 1001, "/home/guest", "/bin/sh");
 
+    // Log in as SUB by default.
+    auth_set_current_user("SUB");
+
     printk(KERN_INFO "AUTH: Pluggable Authentication & SHA-256 Shadow Password engine active\n");
 }