Improve first-open performance of the new Stats screen

[crazytonyli]

Two changes:

• Lazy load some charts.
• Use placeholder graphs, instead of redacted real charts.

Before	After

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📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
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	Build Number	33963
	Version	PR #25931
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[jkmassel]

This seems generally good, but the bar width is pretty different between the placeholder and the actual data – the screenshots on the PR actually show the issue decently well.

Apart from that it seems good – I tested on two devices side-by-side and the loading is very nice and smooth.

[crazytonyli]

but the bar width is pretty different between the placeholder and the actual data

I did not try to match the look, because I don't want to overly complicate the skeleton placeholder view. If we want to match the look, we'd need to pass the selected date range to the placeholder SwiftUI view, which then calculate number of the data points (7 for 7 days, 30 for a month, 12 for a year, plus there is custom date range) and then calculate the width based on that. I feel like that's too much for a placeholder view that's basically a loading indicator.

[jkmassel]

As it stands now, this would be a visual regression – I understand the desire to keep it simple, but I'm not sure it's with the loss of polish. It's especially pronounced on a chart that shows 30 bar lines 😞

[crazytonyli]

@jkmassel I decided to go with another direction (see #25940). If we coupled the new placeholder views in this PR too much with the original chart, it means we'll need to basically re-draw the chart in a different code, which I think is not ideal.