feat: update extension icon with shop storefront design

- Add generate_logo.py script to create icon programmatically
- New shop icon with striped awning and display windows
- Consistent purple color scheme with Nostr Proxy extension
- Update config.json to reference new icon

Closes #150

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config.json

@@ -2,7 +2,7 @@
   "name": "Nostr Market",
   "version": "1.1.0",
   "short_description": "Nostr Webshop/market on LNbits",
-  "tile": "/nostrmarket/static/images/bitcoin-shop.png",
+  "tile": "/nostrmarket/static/images/nostr-market.png",
   "min_lnbits_version": "1.4.0",
   "contributors": [
     {