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>

biome.json

@@ -0,0 +1,34 @@
+{
+	"$schema": "https://biomejs.dev/schemas/2.4.15/schema.json",
+	"vcs": {
+		"enabled": true,
+		"clientKind": "git",
+		"useIgnoreFile": true
+	},
+	"files": {
+		"includes": ["**", "!**/dist"]
+	},
+	"formatter": {
+		"enabled": true,
+		"indentStyle": "tab"
+	},
+	"linter": {
+		"enabled": true,
+		"rules": {
+			"recommended": true
+		}
+	},
+	"javascript": {
+		"formatter": {
+			"quoteStyle": "double"
+		}
+	},
+	"assist": {
+		"enabled": true,
+		"actions": {
+			"source": {
+				"organizeImports": "on"
+			}
+		}
+	}
+}