Define umbrella extension architecture, provenance, and minimal shell

extension/style.css

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+:root { color-scheme: light dark; font-family: system-ui, sans-serif; }
+body { width: 360px; margin: 0; padding: 20px; box-sizing: border-box; }
+h1 { margin-top: 0; }
+nav { display: flex; gap: 6px; }
+button { padding: 8px; cursor: pointer; border: 1px solid GrayText; border-radius: 4px; }
+button[aria-pressed="true"] { border-color: Highlight; outline: 2px solid Highlight; }
+button:focus-visible { outline: 2px dashed Highlight; outline-offset: 3px; }
+main { min-height: 105px; }
+.status { font-size: 0.85rem; color: GrayText; }