Chrome Web Store: add extension icons and prepare store listing

[ianpatrickhines]

Context

Chrome Web Store submission has a hard blocker and missing assets. The extension has no icons — extension/public/ contains only manifest.json, the manifest has no icons field, and dist/ ships none. The store requires a 16/32/48/128 icon set. There are also no store-listing assets (screenshots, promo copy). Permissions scope is already correct (storage, activeTab, alarms, narrow *.nationbuilder.com host permission). The privacy policy content is real and complete but must be live at the canonical domain (see domain-consolidation issue).

Acceptance Criteria

• Icon set added (16/32/48/128) and referenced in manifest.json; build copies them into dist/
• Store listing prepared: title, description, category, at least the minimum required screenshots
• Privacy policy URL is live and linked in the listing
• Manifest version bumped intentionally; npm run build produces a submission-ready zip
• (Human/ops) Submission uploaded to the Chrome Web Store developer dashboard