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/theme.h

@@ -0,0 +1,14 @@
+#pragma once
+#include <lvgl.h>
+
+// Design tokens — single source of truth for UI colors. Anthropic-inspired
+// dark palette, AMOLED-friendly (true black bg).
+#define THEME_BG       lv_color_hex(0x000000)   // screen background
+#define THEME_PANEL    lv_color_hex(0x1f1f1e)   // card/zone fill
+#define THEME_TEXT     lv_color_hex(0xfaf9f5)   // primary text
+#define THEME_DIM      lv_color_hex(0xb0aea5)   // secondary text
+#define THEME_ACCENT   lv_color_hex(0xd97757)   // brand terra-cotta
+#define THEME_GREEN    lv_color_hex(0x788c5d)
+#define THEME_AMBER    lv_color_hex(0xd97757)
+#define THEME_RED      lv_color_hex(0xc0392b)
+#define THEME_BAR_BG   lv_color_hex(0x2a2a28)   // unfilled bar track