fix(emoji): reserve square box for img.emoji to avoid CLS

[imorland]

Fixes #4681

Problem

img.emoji (the inline Twemoji <img> rendered by the formatter) is styled in extensions/emoji/less/forum.less with a height but no width and no aspect-ratio. The Twemoji SVGs are loaded from the jsDelivr CDN and the rendered <img> carries no intrinsic dimensions (the s9e formatter template emits <img alt="…" class="emoji" draggable="false" src="…"/> with no width/height).

With height: 1.5em but width: auto, the browser reserves zero width for each emoji until its SVG downloads, then reflows the surrounding text once each SVG arrives. On emoji-heavy posts this produces visible Cumulative Layout Shift (CLS), especially on first paint / cold CDN cache / throttled mobile connections.

Fix

Every Twemoji glyph is square (1:1), so declaring aspect-ratio: 1 lets the browser derive width: 1.5em immediately from the existing height and reserve the correct box before the SVG loads — eliminating the shift.

This lives in the extension's own stylesheet rather than the generated formatter bundle, so it survives bundle regeneration and is deployment-agnostic (correct for every install).

Testing

1. Post a message containing several emoji (e.g. 😀🎹🔥👍🎉).
2. Load the discussion with the Twemoji SVGs uncached (hard refresh, cache disabled, or network throttling).
3. Before: text reflows horizontally as each SVG finishes loading. After: no layout shift.

Credit to @ekumanov for the report and the suggested fix.