fix: make the container styleable and give it an accessible name

[sidgaikwad]

Fixes #43. Three problems in the same two divs.

1. style could not override flex (the actual bug)

<div id={editorId} ref={containerRef} style={{ ...style, flex: 1 }} />

flex: 1 was written after the spread, so it always won — style={{ flex: 'none' }} or { flex: '0 0 600px' } was silently discarded. Moving it before the spread makes it a default the consumer can override, with zero effect on anyone who doesn't set it.

2. The outer wrapper was unreachable

minHeight was the only thing a consumer could influence on the outer div; its flex: 1 and display: flex were fixed. Dropping the component into a plain block container, a grid cell or a fixed-height panel needed an extra wrapper plus !important. New wrapperStyle prop.

3. No accessible name

I checked what the embed actually renders inside the container on the running demo:

{ "tag": "DIV", "cls": "image-editor-root", "role": null, "aria-label": null }

No landmark, no name, no headings — only svg[role=img] icons. So the entire editing surface was anonymous to assistive tech, and our container contributed nothing but an id.

Added role="region" with an ariaLabel prop defaulting to 'Image editor'.

This one is default-on, so flag it if you'd rather it weren't. A labelled region is a landmark and will show up in screen-reader landmark lists for existing consumers. I went default-on because the measurement above shows there is otherwise no accessible name at all, and region is the conservative choice — role="application" would have changed screen-reader interaction mode, which I did not want to impose on a third-party UI.

Verification

• 2 new tests; 48 total; coverage still 100% statements / branches / functions / lines
• lint, typecheck, build clean
• README props table updated for both new props

Conflict note

The README props-table edit will conflict with #54, which adds a scriptUrl row to the same table. Trivial to resolve; happy to rebase whichever lands second.

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