feat(plugins): let a plugin set coords inside its slot (#1044)

[gloubix]

Closes #1044 — following your comment that position means coords inside "body" or "footer".

What this adds

register() gains an optional third argument:

register(position, element, coords?: { x: number | string; y: number | string })

• With coords → the element renders absolutely positioned at (x, y) inside its slot area. A number is px, a string is any CSS length ("10%", "2rem", …).
• Without coords → exactly today's in-flow behavior. Existing plugins keep working unchanged.

Example from a plugin:

register("body", <MyWidget />, { x: 40, y: "20%" });
register("footer", <StatusBadge />, { x: "80%", y: 0 });

Anchoring

• body: the slot is wrapped in a position: relative container in Index.tsx, so coords are offsets from the body area's top-left.
• footer: <PluginSlot position="footer" /> is now mounted inside Footer's fixed root Grid (footer was in the position enum but no slot was rendered anywhere) — coords anchor to the footer area.

Notes

• Plugins.tsx reuses the shared RegisterPluginFn type so remote bundles see the new signature (and can also target options, which the local type copy didn't include).
• 4 files, +74/−34. tsc and vp lint clean.
• If by "coords" you had something else in mind (grid cells, anchor presets like top-left/center…), say the word and I'll adjust — the registry shape supports swapping the placement semantics without touching the plugin ABI again.