feat: initial @unom/app-ui — shared app-level blocks, starting with the footer

Composite UI blocks shared across the unom/punktfunk apps, one layer up from
@unom/ui (primitives). Follows the @played/app-ui pattern: per-block subpath
exports, tsdown ESM+dts build, themed through @unom/style semantic tokens.

The ./footer block exports a presentational FooterView (link sections, socials,
tagline) with a resolveHref hook so the docs can rebase root-relative links onto
the marketing-site origin. Both the punktfunk marketing site and docs render it,
so the footer no longer drifts between hand-mirrored copies.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tsdown.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from "tsdown";
+
+export default defineConfig({
+	// Keyed entry so the output keeps its subpath (dist/footer/index.mjs),
+	// matching the `./footer` subpath export declared in package.json — leaving
+	// room for more blocks (./auth, ./page, …) without a breaking export move.
+	entry: { "footer/index": "./src/footer/index.tsx" },
+	format: "esm",
+	dts: true,
+});