Fix a Firefox crash from a WebKit-only scrollbar selector

[JedimEmO]

The deployed site renders a blank page in Firefox on every route — https://jedimemo.github.io/dwind/examples/#/

Cause

slim_scrollbar is applied to the app root (lib.rs:61), and it used [&::-webkit-scrollbar-thumb:hover]:. Firefox tolerates the unknown pseudo-element on its own but rejects it with a pseudo-class appended, so insertRule throws — and dominator panics rather than skipping the rule:

// dominator/src/dom.rs:1691
} else {
    // TODO maybe make this configurable
    panic!("selectors are incorrect:\n  {}", failed.join("\n  "));
}

That kills the app mid-construction. Firefox before the fix:

hasHeader: false, hasMain: false, h1: null, bodyText: "", styleSheets: 12

after:

hasHeader: true,  hasMain: true,  h1: "Styling forged at compile time.", styleSheets: 21

This is a regression from #23. A raw stylesheet drops an unparseable rule silently, so the same selector was harmless in styles.rs. Moving a selector into a class makes an unsupported one fatal — that trade wasn't obvious to me when I made it, and it's the kind of thing only a second engine catches.

Fix

The pseudo-elements aren't needed: Firefox has supported scrollbar-width/scrollbar-color since 64, Chrome since 121. The mixin now uses the standard properties alone.

Bisect

A disposable probe narrowed it precisely — the bare pseudo-element is fine, only the :hover combination throws:

selector	Firefox
[&::-webkit-scrollbar]:[width:10px]	ok
[&::-webkit-scrollbar-track]:[background:transparent]	ok
[&::-webkit-scrollbar-thumb]:[background:#26262C]	ok
[&::-webkit-scrollbar-thumb:hover]:[background:#3A3A44]	FAIL
[scrollbar-color:#26262C transparent]	ok

Guarding against a repeat

Adds browser tests pinning every selector shape the example applies — the spotlight ::before/::after pair with its masks, the reveal cascade's parent-state and :nth-child variants, the marquee's [&:hover > *], and the scrollbar properties. CI runs both Firefox and Chrome, so an engine that rejects any of them now fails the build rather than the page.

Also documents the hazard in the changelog: with variants now able to express selectors a stylesheet used to hold, an unsupported one is fatal instead of ignored.

Verification

• Reproduced and fixed in real Firefox via geckodriver, across five routes
• Full wasm suite under both Firefox and Chrome
• cargo fmt --all -- --check, host tests, clippy

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