Add welcome screen at the start of first-time setup, before touch calibration

Shows a new FirstRunWelcomeScreen right at the very beginning of the
first-time setup flow (isFirstRun), before touch calibration even
runs: hard-coded English title (no language chosen yet at this point),
the splash screen's radar reticle centered below it, and a large
black/green-outlined "Start" button whose text breathes between a
darker and a lighter, slightly whitish green (~2.5s cycle) so it's the
one element that visibly stands out against the otherwise uniform
green UI. The button intentionally stays unfilled (black) so the
twinkling MenuStars background shows through it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/first_run_welcome_screen.h

@@ -0,0 +1,7 @@
+#pragma once
+#include <Arduino.h>
+#include <TFT_eSPI.h>
+
+namespace FirstRunWelcomeScreen {
+    void run(TFT_eSPI& tft);
+}