fix: bump chrome-manifest.json to 1.5.6

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vveerrgg

src/chrome-manifest.json

@@ -3,7 +3,7 @@
     "default_locale": "en",
     "name": "__MSG_extension_name__",
     "description": "__MSG_extension_description__",
-    "version": "1.5.5",
+    "version": "1.5.6",
     "short_name": "NostrKey",
     "author": "Humanjava Enterprises Inc",
     "homepage_url": "https://nostrkey.com",