Scroll speeds below 1 dot per second

scroll_speed is now a float (0.05-30 dots per second, e.g. 0.25 = one dot
every 4 s). Page format 5; version 4 pages are converted, version 3 goes
through version 4.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/logic/page_render.cpp

@@ -1,5 +1,6 @@
 #include "logic/page_render.h"
 
+#include <math.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -157,13 +158,15 @@ static uint32_t drawTextElement(const PageElement &e, const char *text, uint32_t
 
 	// Scrolls from right to left, the next round follows after SCROLL_GAP dots
 	uint32_t period = width + SCROLL_GAP;
-	uint64_t steps = (uint64_t)now_ms * e.scroll_speed / 1000;
+	// Step n starts at n * 1000 / speed ms (the speed can be below 1 dot per second)
+	double ms_per_step = 1000.0 / e.scroll_speed;
+	uint64_t steps = (uint64_t)(now_ms / ms_per_step);
 	int offset = (int)(steps % period);
 	drawText(bitmap, font, left - offset, e.y, text, mode, left, left + area);
 	drawText(bitmap, font, left - offset + (int)period, e.y, text, mode, left, left + area);
 	// Until the next step
-	uint64_t next_step_ms = ((steps + 1) * 1000 + e.scroll_speed - 1) / e.scroll_speed;
-	uint32_t wait = (uint32_t)(next_step_ms - now_ms);
+	uint64_t next_step_ms = (uint64_t)ceil((steps + 1) * ms_per_step);
+	uint32_t wait = next_step_ms > now_ms ? (uint32_t)(next_step_ms - now_ms) : 0;
 	return wait ? wait : 1;
 }