Store links for the extension in the README - #68

The extension is live in both stores. The README now says so:

• Two badges in the header row — Chrome Web Store and Firefox Add-ons, each showing the published version.
• The extension section starts with the two install links; the unpacked-build path stays below as the store-free alternative, replacing "It is not in either store yet."

Both store pages and both badge endpoints answer 200; the badges currently report v1.0.0 on each store.

🤖 Generated with Claude Code