chore(updater): set real minisign public key

Replaces PLACEHOLDER_PUBLIC_KEY with the real minisign public key
generated via 'tauri signer generate'. The corresponding private key
and password are stored locally; they must be added to GitHub Actions
secrets (TAURI_SIGNING_PRIVATE_KEY and TAURI_SIGNING_PRIVATE_KEY_PASSWORD)
before the first signed release.

Minisign public keys are intended to be public and embedded in the
binary; the updater uses this key to verify the signature of the
downloaded update bundle.

cc

src-tauri/tauri.conf.json

@@ -41,7 +41,7 @@
       "endpoints": [
         "https://github.com/vicrodh/qbz/releases/latest/download/latest.json"
       ],
-      "pubkey": "PLACEHOLDER_PUBLIC_KEY"
+      "pubkey": "dW50cnVzdGVkIGNvbW1lbnQ6IG1pbmlzaWduIHB1YmxpYyBrZXk6IDNGN0ZDRTZERUJCQjk1ODYKUldTR2xidnJiYzUvUDMvNHpvWFpmQmQrcGVlNUt3N2g1L2dVSUU3QjlHaUw0N2R2VVNOcE5HTG0K"
     }
   },
   "bundle": {