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

userland/lazybox/lazybox.c

@@ -709,6 +709,8 @@ static int applet_free(int argc, char** argv) {
            total_kb, used_kb, free_kb, free_kb);
     printk("Heap:       %8llu    %8llu    %8llu\n",
            kmalloc_get_total() / 1024, kmalloc_get_used() / 1024, kmalloc_get_free() / 1024);
+    printk("            (dynamic kernel heap, grown on demand %llu time(s))\n",
+           (unsigned long long)heap_get_grow_count());
     return 0;
 }
 
@@ -1137,7 +1139,7 @@ static int applet_aead(int argc, char** argv) {
 static int applet_whoami(int argc, char** argv) {
     (void)argc; (void)argv;
     const user_account_t* u = auth_get_current_user();
-    printk("%s\n", u ? u->username : "root");
+    printk("%s\n", u ? u->username : "SUB");
     return 0;
 }
 
@@ -1652,7 +1654,7 @@ static int applet_ssh(int argc, char** argv) {
         return 1;
     }
 
-    char username[32] = "root";
+    char username[32] = "SUB";
     char host[64] = "localhost";
     uint16_t port = 22;
 
@@ -1774,7 +1776,7 @@ static int applet_logread(int argc, char** argv) {
 }
 
 static int applet_su(int argc, char** argv) {
-    const char* target = (argc >= 2) ? argv[1] : "root";
+    const char* target = (argc >= 2) ? argv[1] : "SUB";
     if (auth_set_current_user(target) == 0) {
         printk(KERN_INFO "Switched to user '%s'\n", target);
         return 0;
@@ -1784,7 +1786,7 @@ static int applet_su(int argc, char** argv) {
 }
 
 static int applet_passwd(int argc, char** argv) {
-    const char* user = (argc >= 2) ? argv[1] : "root";
+    const char* user = (argc >= 2) ? argv[1] : "SUB";
     const char* new_pass = (argc >= 3) ? argv[2] : "password";
     if (auth_change_password(user, new_pass) == 0) {
         printk(KERN_INFO "passwd: password updated successfully for '%s'\n", user);