Update panel size max for sidebar

[JedWatson]

This isn't a particularly big change or comprehensive fix or anything, but it's been annoying me that when I resize windows, the navigation sidebar always ends up too big and then I have to drag it smaller.

So after some tinkering (you're right @jossmac this is really fiddly) I ended up just reducing the maximums to make it "feel better". Let's shrink it and see if anyone comes up with a use-case for it to be meaningfully wider than the new max?

Otherwise it really eats into content entry space when you resize the window, on a normal 13" screen viewport.

Before:

After:

[changeset-bot]

🦋 Changeset detected

Latest commit: 52da8cd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 8 packages

Name	Type
@keystatic/core	Patch
@example/astro	Patch
@example/localization	Patch
@example/next-app	Patch
@keystatic/remix-test-app	Patch
keystatic-docs	Patch
@keystatic/templates-astro	Patch
@keystatic/templates-nextjs	Patch

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[jossmac]

Yeah, the percentage API isn't appropriate. Unfortunately the author will not support pixel dimensions: 46, 47, 78, 114...

I need to roll a bespoke implementation, but it's not priority.