OSD: Fix size calculation for progress value

Use a common function for formatting the percent, so placeholder 100% label will use the exact formatting used by the label next to the progress bar.

Also use font advanceWidth, which is the appropriate means of calculating the bounding box of text in this case. Also ceil it to avoid subpixel alignment.

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My 100% volume label no longer wraps awkwardly.

Does this need cherry-picking to 5.27 or is this only broken on Qt 6? We’re not adding new strings here, just removing and shuffling existing ones, this is fine for stable.