fix(demo): make the page responsive and follow the theme control

[sidgaikwad]

Fixes #39.

1. The demo was not responsive

styles.css contained no @media query at all. .sidebar was a hard width: 240px; flex-shrink: 0, so on a 375px phone the editor got ~135px — and because useState(true) opened the sidebar by default, the first paint on mobile was the broken one.

Before (375×812): sidebar occupied ~64% of the viewport, the editor was a sliver, and the editor's own Save/Cancel controls were off-screen entirely.

After (same viewport): sidebar starts closed, editor gets the full 375px, tool rail and Save/Cancel all reachable.

Below 768px the sidebar now overlays the editor (position: absolute, min(280px, 85vw)) with a dismissable backdrop, rather than squeezing it. App.tsx starts it closed at those widths via matchMedia, keyed to the same breakpoint.

Measured in the running demo:

	375×812	1280×800
sidebar open by default	false	true
sidebar position	absolute (overlay)	static (in flow)
editor width, sidebar closed	375	—
editor width, sidebar open	375 (unchanged — overlaid)	1040
backdrop	shown	display: none

Desktop layout is unchanged.

2. The theme control didn't theme the page

The Light/Dark selector drove options.theme, but the surrounding page was hardcoded — html/body light, .topbar and .sidebar dark, .preview white. Switching to dark gave a dark editor inside light chrome; switching to light still left a permanently dark sidebar. Since the demo is what people look at to judge whether theming works, it undersold it.

All chrome colours now come from CSS custom properties keyed off a data-theme attribute that App.tsx sets from the same theme state the editor receives:

themeAttr:  light  ->  dark
body bg:    rgb(255,255,255)  ->  rgb(15,17,21)
topbar bg:  follows to rgb(22,24,29)

Verification

Ran against the live demo at both viewports; tsc --noEmit and vite build clean; Prettier clean.

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[ivoIturrieta]

In landscape mobile view, the saved preview grows taller than the viewport and its Close/Download controls disappear above the screen. Please constrain the preview height and keep its header accessible.