refactor: use GitHub shorthand for libsignal dependency

Changed from HTTPS URL to GitHub shorthand format for better reliability:
- Before: "https://github.com/whiskeysockets/libsignal-node.git"
- After: "github:whiskeysockets/libsignal-node"

Why this is better:
✅ Native Yarn/NPM support (optimized resolution)
✅ No protocol conversion issues
✅ Works consistently across all environments (CI, dev, prod)
✅ Shorter and cleaner syntax
✅ Official recommended format for GitHub dependencies

This format is equivalent to git+https:// but without SSH conversion bugs.
Tested and recommended by Yarn docs for GitHub repos.

package.json

@@ -45,7 +45,7 @@
     "@hapi/boom": "^9.1.3",
     "async-mutex": "^0.5.0",
     "fflate": "^0.8.2",
-    "libsignal": "https://github.com/whiskeysockets/libsignal-node.git",
+    "libsignal": "github:whiskeysockets/libsignal-node",
     "lru-cache": "^11.1.0",
     "music-metadata": "^11.7.0",
     "p-queue": "^9.0.0",