Centralize colors in theme.h and propagate true-black background

- Extract design tokens to firmware/src/theme.h (THEME_BG, THEME_PANEL,
  THEME_TEXT, etc.); ui.cpp aliases COL_* to the THEME_* tokens
- Splash container + canvas COL_EMPTY now use THEME_BG / 0x0000 instead
  of the hardcoded 0x0f0f0f / 0x10A2 dark grey
- tools/convert_to_c.js maps "transparent" palette entries to 0x0000
  (true black) so regenerated splash_animations.h matches the new bg
- Re-regenerate splash_animations.h
- Convert logo to RGB565A8 (alpha plane) so it composites against any
  background — no more grey halo around the upper-left logo on screens
  where the bg differs from the previous baked-in 0x18E3 color

firmware/src/splash.cpp

@@ -1,5 +1,6 @@
 #include "splash.h"
 #include "splash_animations.h"
+#include "theme.h"
 #include <Arduino.h>
 #include <esp_heap_caps.h>
 
@@ -10,7 +11,7 @@
 #define CANVAS_H     (GRID * CELL)
 
 // Background fallback when palette is missing
-#define COL_EMPTY    0x10A2  // dark bg (#0f0f0f)
+#define COL_EMPTY    0x0000  // true black (matches THEME_BG)
 
 LV_FONT_DECLARE(font_styrene_28);
 
@@ -57,7 +58,7 @@ void splash_init(lv_obj_t *parent) {
     splash_container = lv_obj_create(parent);
     lv_obj_set_size(splash_container, 480, 480);
     lv_obj_set_pos(splash_container, 0, 0);
-    lv_obj_set_style_bg_color(splash_container, lv_color_hex(0x0f0f0f), 0);
+    lv_obj_set_style_bg_color(splash_container, THEME_BG, 0);
     lv_obj_set_style_bg_opa(splash_container, LV_OPA_COVER, 0);
     lv_obj_set_style_border_width(splash_container, 0, 0);
     lv_obj_set_style_pad_all(splash_container, 0, 0);