Refresh the examples browser UI

[willeastcott]

What changed

A visual and functional refresh of the examples browser (examples/index.html + css/browser.css + js/*.mjs):

• Dark PlayCanvas-branded theme built on CSS custom properties: near-black surfaces (#121212 / #1a1a1a), PlayCanvas orange (#f60) accent, themed scrollbars, :focus-visible outlines, dark QR dialog (the QR image sits on a white tile so it stays scannable), dark hamburger/drawer on mobile, and matching PWA manifest / theme-color meta.
• Search: new search box filters by example name or category. / focuses it from anywhere, Esc clears (then blurs), Enter loads the first match, empty categories hide, and a "No examples found" state covers zero matches.
• Category grouping: the sidebar is grouped into 8 sections (Graphics, Animation, Physics, Sound, UI & Text, Gaussian Splatting, XR, Showcases). Ad-hoc name prefixes ("Splat:", "UI:", "Game:") are dropped now that group headers give context. Paths are unchanged, so existing deep links keep working.
• Two examples added to the list that existed on disk but were never listed: AR Hand Gestures and 2D Screen (27 total). The three spinning-cube getting-started variants remain unlisted but reachable by URL, as before.

Bugs fixed along the way

• Back/forward desync: setting iframe.src pushed iframe navigations into the joint browser history, interleaved with the pushState hash entries — so Back rewound the iframe while the sidebar/hash stayed put. The iframe now navigates via location.replace(), which keeps history to hash entries only; Back/Forward now keeps the URL, active row, and iframe in sync. The initial URL update also uses replaceState so the first Back press isn't dead.
• Mobile overlay never worked: .sidebar-overlay had no CSS (zero-size div), so tapping outside the drawer couldn't close it. Now styled and functional.
• QR dialog listener leak: a new backdrop-click handler was attached on every open; now attached once.
• Arrow-key navigation now walks only visible (unfiltered) links and is safe with zero results.

Notes for reviewers

• The default landing example (no hash) is now Basic Shapes (first entry after the category reorder) instead of Animation — trivially adjustable via group order if undesired.
• The active row is white-on-#f60 (~2.9:1 contrast, consistent with the previous UI and playcanvas.com). If AA contrast is preferred, the alternative is orange text on an orange tint.
• Verified locally: deep links (including both new examples and bogus-hash fallback), all search/keyboard behaviors, history round-trips, QR modal repeated open/close, mobile drawer at 375x812, and npm run lint.

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