bootloader: fix duplicate "Autobooting in X seconds" line

The countdown row was derived as list_y - 4, which landed one row too low
(on the "Use Up/Down" line) instead of on the header line printed before the
loop. The in-loop countdown therefore rendered a second "Autobooting in X
seconds" copy directly below the static header, showing the phrase twice.

Capture the header's row with get_text_position() right before printing it, so
the loop updates that exact line in place — one countdown line, no stale copy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -221,6 +221,11 @@ fn select_mode(
     let mut countdown = AUTOBOOT_SECONDS;
     let mut countdown_active = true;
 
+    // Capture the countdown line's row now, so the in-loop countdown updates it
+    // in place. Deriving it from list_y with a fixed offset landed one row too
+    // low (on the "Use Up/Down" line), leaving this header as a stale second
+    // "Autobooting in X seconds" copy above the live one.
+    let countdown_y = os.get_text_position().1;
     os.set_text_color(TextColor::Yellow);
     println!(
         "  Autobooting in {} seconds (press any key to cancel)",
@@ -240,7 +245,6 @@ fn select_mode(
     println!();
 
     let (list_x, list_y) = os.get_text_position();
-    let countdown_y = list_y.saturating_sub(4);
 
     loop {
         for (i, entry) in entries.iter().enumerate() {