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

tools/convert_to_c.js

@@ -26,7 +26,7 @@ function safeIdent(s) {
 }
 
 function hexToRgb565(hex) {
-  if (!hex || hex === 'transparent') return 0x10A2;  // dark bg
+  if (!hex || hex === 'transparent') return 0x0000;  // dark bg
   let h = hex.replace('#', '');
   if (h.length === 3) h = h.split('').map(c => c + c).join('');
   const r = parseInt(h.substr(0, 2), 16);
@@ -36,7 +36,7 @@ function hexToRgb565(hex) {
 }
 
 function paletteToRgb565(palette) {
-  const out = new Array(PALETTE_SIZE).fill(0x10A2);
+  const out = new Array(PALETTE_SIZE).fill(0x0000);
   for (let i = 0; i < palette.length && i < PALETTE_SIZE; i++) {
     out[i] = hexToRgb565(palette[i]);
   }