feat: let the editor fit a narrow, phone-sized container

[alirezamhi]

What

• min-width: 0 on both container divs, so the editor can never stretch the page it is embedded in.
• className and wrapperClassName on the container and the wrapper.
• A README "Sizing and mobile" section documenting the three rules for a small screen.

Why

The component renders display: flex with a flex: 1 container, and a flex item defaults to
min-width: auto — a floor at its content's min-content width, which it refuses to shrink below.
This editor's content is far wider than any phone: the sticker and shape panels lay hundreds of
tiles out in rows that never wrap, measured at 13,634px of min-content.

In the embed's default layout that floor stays hidden, because the panel is shrink-0 with
max-width: calc(100% - 4.5rem): its width is capped by its parent, so the huge content minimum
never becomes the container's floor. But a phone has no room for a 288px panel beside the canvas, so
any mobile layout has to stack the panel. The moment it does, the panel's width comes from the
cross-axis stretch, its 13,634px minimum becomes the container's, the container becomes 13,634px
wide and the page grows a horizontal scrollbar.

Measured in this repo's own demo (pnpm --dir demo dev), 375px viewport, sticker panel open, with
only the two layout rules a phone layout needs (flex-direction: column on the panel row,
width: 100% on the panel):

container min-width	container	documentElement.scrollWidth
auto (current)	13,671px	13,671px
0 (this PR)	375px	375px

36 viewports wide: the tool rail is pushed off-screen and about 1 of the 8 tool icons stays
reachable. With the fix the container holds at the viewport width and the sticker rows keep their
13,634px — scrolling inside their own panel, which is how the embed already treats them.

Also: the class names

Rearranging the embed's chrome for a phone needs a breakpoint, and a breakpoint needs a selector: a
media query cannot be expressed in the style / wrapperStyle objects the component already
accepts, and inline styles cannot express one either. className / wrapperClassName are that
hook; editorId covers the id-based case.

Notes

• min-width: 0 is written before the spread, matching the existing "flex first: a default the
  consumer's style can override" contract from the flex default — so it is not a behaviour change
  for a container already wide enough to hold the editor, and style / wrapperStyle can restore
  the previous behaviour.
• Tests: two added (the default, and an explicit minWidth overriding it). Suite at 76 passed, and
  the 100% coverage thresholds still hold.
• Locally green: pnpm lint, pnpm typecheck, pnpm build, pnpm test:coverage, and the demo's
  typecheck / build.

Fixes #94

[vercel]

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[alirezamhi]

Before / after at a 375px viewport, sticker panel open, captured in a real browser from this repo's demo (pnpm --dir demo dev) with only the two layout rules a phone layout needs:

min-width: auto (current)	min-width: 0 (this PR)
container 13,671px — the page scrolls sideways, the tool rail is pushed off-screen	container 375px — the 13,634px of sticker tiles scroll inside the panel

Same harness, same run: document.documentElement.scrollWidth goes 13,671 → 375.