Fixes #4991.

The bug

A local SVG that states no width/height, only a viewBox — the shape every
rich/Textual/mermaid-style export produces — renders as nothing in the editor.
Reproduced on 0.20.0-rc.4 with the exact file from the issue: the <img> loads
fine (.mu-image-success, complete === true, naturalWidth 300), yet its
layout box measures 0x0.

Why

Such an image has no intrinsic size, so CSS 2.1 §10.3.2 resolves its used width
against its containing block. .mu-inline-image and .mu-image-container are
both inline-block, i.e. shrink-to-fit: the container's width comes from the
image, the image's from the container, and Chromium breaks the cycle at 0. The
same SVG in ordinary block flow (or on GitHub) lays out at 300x136.

This is not a regression — the legacy engine's .ag-inline-image /
.ag-image-container chain is identical, which is why the issue was filed
against 0.19.1. An SVG that states its own size has always worked.

CSS alone can't fix it: there is no selector for "replaced element with no
intrinsic size", and max-width: none, width: fit-content, display: block
on the container and 100cqw were each measured in the real app — all still 0.

The fix

When a loaded image's measured size is the browser's fallback (the CSS default
object size, 300x150 contained to the image's ratio — what loadImage already
reports), pin that width on the <img>, so the shrink-to-fit container has
something to shrink to. The height stays automatic, so max-width: 100% still
scales the image by its ratio when the editor is narrow.

• Images with an intrinsic size are untouched.
• Images whose markdown/HTML attributes state a size are untouched.
• An image that really is 300px wide is pinned to its own intrinsic width, which
  renders identically.

Applied in both render paths: loadImageAsync's success handler (first load)
and image.ts (every re-render rebuilds the <img> from vnodes).

Verification

Real app (electron-vite build of this branch), the issue's own SVG plus a
matrix of local files:

Sizes survive a re-render, and narrowing the editor still scales every image by
its ratio (max-width: 100% unaffected).

Tests: new e2e spec fails on develop (0) and passes with the fix (300x150);
unit tests cover the helper and both render paths. vitest run 1911 passed,
eslint clean, tsc --noEmit clean, madge --circular clean, and the image
e2e suites (drag, image tools, reference images, inline) all pass.

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