Create a single file called index.html containing a complete broadcast countdown timer for use as an OBS browser source, with all HTML, CSS and JavaScript in that one file and no external libraries.

Layout: the top of the page is a fixed stage exactly 1920 pixels wide and 1080 pixels tall with a fully transparent background. This stage is what OBS will capture. Centred on the stage, show a very large countdown timer in MM:SS format (switch to H:MM:SS if an hour or more remains) using bold tabular figures with a subtle dark outline or drop shadow so it stays readable over video. Directly below the 1080-pixel line, add an operator control panel on a dark visible background — this area sits outside the OBS crop and is only seen by the operator in a normal browser or the OBS Interact window. Label the boundary clearly with a thin line and the caption "OBS crop line: nothing below this is broadcast".

Controls, in the panel: number inputs for starting minutes and seconds; Start, Pause and Reset buttons; quick preset buttons for 5, 10 and 15 minutes; +1 min and −1 min buttons that adjust the time even while running; and a toggle button that switches the stage background between transparent and a grey checkerboard for previewing transparency.

Behaviour: the timer must be drift-free — compute remaining time from a target end timestamp rather than decrementing a counter, so it stays accurate over long runs. While more than 60 seconds remain the digits are white; at 60 seconds or less they turn amber; at 10 seconds or less they turn red. When the timer reaches 00:00 it stops and the digits change to a muted grey. Reset returns to the value in the number inputs and restores the white colour. Pause freezes the remaining time exactly; Start resumes from it. Guard against invalid input (negative numbers, blank fields) by treating them as zero.

Keep the code clearly commented so a beginner can ask follow-up questions about how each part works.