feat: gallery viewer: big resizable stage, pages (templates) as a category, better cards, lib PreviewStage #557

The /gallery/ cards are too small to judge a component, can't be expanded, and full pages (templates) live in a separate /templates/ list.

Build:

1. Demo viewer at /gallery// (deep-linkable) opened from every card: resizable stage (390/768/1280/fill + ResizablePanels handle), light/dark + preset per stage, Code tab with copy, Pick, Copy prompt, prev/next arrow keys, reduced-motion aware, replay for motion demos.
2. Templates become a first-class gallery category "pages" (thumbnail card, iframe-isolated stage at real width) plus beta pages marked as beta.
3. Bigger cards: larger preview, description clamped to 2 lines, Open on hover/focus.
4. Lib: PreviewStage / DeviceFrame component for consumers' own showcases (browser tests).
5. A11y: keyboard + SR, axe clean open/closed, no CLS, lazy-mount off-screen previews.