feat: add a placeholder slot for the embed loading gap

[sidgaikwad]

Fixes #37.

Problem

Between mount and the editor being ready, the component rendered an empty container. On a cold CDN cache that's the full embed.js request plus the versioned-bundle hop showing as a blank box at minHeight: 500.

There was no supported way to fill it: onLoad only fires after success, so consumers had to render their own absolutely-positioned overlay from first paint, tear it down in onLoad, know about the internal two-div layout — and it still didn't cover the failure path.

Change

<ImageEditor image={url} placeholder={<Spinner />} />

Two design decisions worth your attention:

• It's an overlay sibling, not a child of the mount container. The embed owns that container's DOM (it renders div.image-editor-root into it), so React must not reconcile children in and out from under it.
• It stays visible when the mount fails. The issue flagged this as an open question. A failed load ending on a blank box is the worst outcome, so the placeholder persists and consumers pair it with onError to swap in a failure message. Documented in the README and the prop's doc comment.

position: relative is applied to the wrapper only while a placeholder is actually showing, so existing layouts are untouched.

Verification

Beyond the unit tests, I wired it into the demo and pointed scriptUrl at a non-existent file to hold the failure state open.

Before — embed fails to load, entire editor area is blank white:

status bar reads Error: Failed to load the image editor embed script: .../DOES-NOT-EXIST.js, content area completely empty

After — same failure, with a spinner and "Loading the image editor…" centred in the area.

The demo scaffolding was reverted; this PR touches only src/, test/ and the README.

• 4 new tests (shown until mount, absent when the prop is, survives a failed mount, returns during a remount)
• 50 tests; coverage still 100% statements / branches / functions / lines
• lint, typecheck, build clean

Conflict note

Touches the same render block as #55, and the same README props table as #54.

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