Add screenshot capture via Joy-Con Capture button

- source/screenshot.c, screenshot.h: take_screenshot() saves framebuffer as BMP
- Path: sd:/switch/screenshot/omninx_installer_YYYYMMDD_HHMMSS.bmp (RTC timestamp)
- 3s cooldown, backlight flash on capture; TegraExplorer-style BGR flip
- main.c: call take_screenshot() when jc->cap in all joycon poll loops

Made-with: Cursor

source/screenshot.h

@@ -0,0 +1,13 @@
+/*
+ * Screenshot support for OmniNX Installer Payload
+ * Based on TegraExplorer (AllgemeinerProblemLoeser) TakeScreenshot implementation.
+ */
+
+#pragma once
+
+#include <utils/types.h>
+
+/* Take a screenshot of the current framebuffer and save as BMP to SD.
+ * Triggered by Capture button (jc->cap). Uses 3-second cooldown.
+ * Saves to sd:/switch/screenshot/omninx_installer_YYYYMMDD_HHMMSS.bmp */
+void take_screenshot(void);