feat: lvt-scroll-away support data-lvt-target="window" / page-level scroll

[adnaan]

Context

client#102 shipped `lvt-scroll-away="top"` (PR #103), enabling scroll-to-top buttons targeting a discrete scrollable container via `data-lvt-target="#scroll-id"`. However, page-level scroll (most commonly: the document body) cannot be addressed by the current target syntax.

Requirements

`lvt-scroll-away` should accept a sentinel target meaning "the viewport/page scroll":

```html
Scroll to top
```

Implementation considerations:

• `resolveTarget` (in `dom/reactive-attributes.ts`) currently supports only `#id` and `closest:sel` — needs a `window` / `document` keyword
• Scroll source for window: listen on `window` (the only element that fires page scroll events), read `window.scrollY` for top-edge distance and `document.documentElement.scrollHeight - window.innerHeight - window.scrollY` for bottom-edge distance
• The `scroll-away.ts` handler currently does `target.scrollTop`/`target.scrollHeight` — needs branching when target is window
• `scroll-away.ts` event listener attaches to `target` directly — needs branching to attach to `window` when target is window

Use case

The original Pattern #10 (Infinite Scroll) intent in livetemplate/livetemplate examples uses page-level scroll, so a page-level scroll-to-top button is the natural fit — but currently impossible without wrapping the list in an `overflow: auto` container, which changes the pattern's UX.

Workaround

Until this lands, use a discrete scrollable container with explicit `overflow: auto` and `max-height` styling, then target it by id.

Related

• client#102 (primitive — shipped in PR feat: lvt-scroll-away top edge for scroll-to-top buttons #103)
• livetemplate/livetemplate `docs/proposals/patterns.md:2497` (Session 7 deferred note)