ngui: get rid of the ambiguous ui header file

functions declared in ui.h were actually used by ui.c, the opposite of
what such a header is expected to contain. these functions are defined
in zig code and declarations are better off in the same ui.c file.

this is just a quick clean up before porting some C code to zig.

build.zig

@@ -54,7 +54,7 @@ pub fn build(b: *std.build.Builder) void {
     ngui.defineCMacroRaw(b.fmt("NM_DISP_VER={}", .{disp_vert}));
     ngui.defineCMacro("LV_CONF_INCLUDE_SIMPLE", null);
     ngui.defineCMacro("LV_TICK_CUSTOM", "1");
-    ngui.defineCMacro("LV_TICK_CUSTOM_INCLUDE", "\"ui.h\"");
+    ngui.defineCMacro("LV_TICK_CUSTOM_INCLUDE", "\"lv_custom_tick.h\"");
     ngui.defineCMacro("LV_TICK_CUSTOM_SYS_TIME_EXPR", "(nm_get_curr_tick())");
     switch (drv) {
         .sdl2 => {

src/ui/c/lv_custom_tick.h

@@ -0,0 +1,16 @@
+#ifndef NM_UI_H
+#define NM_UI_H
+
+/**
+ * this file exists to satisfy LV_TICK_CUSTOM_INCLUDE.
+ */
+
+#include <stdint.h>
+
+/**
+ * returns elapsed time since program start, in ms.
+ * rolls over when overflow occurs.
+ */
+uint32_t nm_get_curr_tick();
+
+#endif

src/ui/c/ui.c

@@ -1,12 +1,31 @@
+/**
+ * function declarations with nm_ prefix are typically defined in zig,
+ * while extern'ed function definitions with nm_ prefix are typically called from zig.
+ */
+
 #define _DEFAULT_SOURCE /* needed for usleep() */
 
 #include "lvgl/lvgl.h"
 
-#include "ui.h"
-
 #include <stdlib.h>
 #include <unistd.h>
 
+/**
+ * initiates system shutdown leading to poweroff.
+ */
+void nm_sys_shutdown();
+
+/**
+ * invoken when the UI is switched to the network settings tab.
+ */
+void nm_tab_settings_active();
+
+/**
+ * initiate connection to a wifi network with the given SSID and a password.
+ * connection, if successful, is persisted in wpa_supplicant config.
+ */
+int nm_wifi_start_connect(const char *ssid, const char *password);
+
 static lv_style_t style_title;
 static lv_style_t style_text_muted;
 static lv_style_t style_btn_red;

src/ui/c/ui.h

@@ -1,28 +0,0 @@
-#ifndef NM_UI_H
-#define NM_UI_H
-
-#include <stdint.h>
-
-/**
- * returns elapsed time since program start, in ms.
- * rolls over when overflow occurs.
- */
-uint32_t nm_get_curr_tick();
-
-/**
- * initiates system shutdown leading to poweroff.
- */
-void nm_sys_shutdown();
-
-/**
- * invoken when the UI is switched to the network settings tab.
- */
-void nm_tab_settings_active();
-
-/**
- * initiate connection to a wifi network with the given SSID and a password.
- * connection, if successful, is persisted in wpa_supplicant config.
- */
-int nm_wifi_start_connect(const char *ssid, const char *password);
-
-#endif