Harden standalone mode watchdog and SD-card mount lifecycle

[Copilot]

This addresses the PR #208 review-thread findings around standalone mode reliability and SD-card config validation. The update keeps scope to the commented areas: watchdog behavior in standalone state, FatFS mount lifetime/prototypes, and API/docs consistency.

• Standalone state machine

  • Clear watchdog in Standalone() idle loop to match Standby() behavior and avoid resets while waiting for UART traffic.

• Standalone SD mount lifecycle

  • Replace stack-scoped FatFS mount object with static standalone-scoped storage to avoid dangling mount pointers across function return.
  • Keep explicit unmount path and expose SDCARD_standalone_unmount() in the public header.
  • Ensure declaration visibility where used.

• Config header validation + API contract clarity

  • Keep strict 5-byte "PSLAB" magic check and make intent explicit via STANDALONE_MAGIC_LEN + memcmp.
  • Use byte buffer type for raw header reads.
  • Align sdcard.h docs with implementation (SUCCESS/FAILED, exact 5-byte magic semantics).

#define STANDALONE_MAGIC_LEN 5
static FATFS s_standalone_drive;

if (f_mount(&s_standalone_drive, SDCARD_DRIVE, 1) != FR_OK) {
    return FAILED;
}

uint8_t buf[STANDALONE_MAGIC_LEN] = {0};
if (f_read(&file, buf, STANDALONE_MAGIC_LEN, &bytes_read) != FR_OK
    || bytes_read != STANDALONE_MAGIC_LEN) {
    f_close(&file);
    return false;
}
return memcmp(buf, "PSLAB", STANDALONE_MAGIC_LEN) == 0;