fix(ci): rename entitlements file — vpk requires .entitlements extension

Nightly run 29093119570 failed:
  --signEntitlements does not have an .entitlements extension
vpk validates the file extension. Rename
resources/entitlements.mac.plist -> entitlements.mac.entitlements
and update all references (electron-builder config, sign-macos-binaries.sh,
both workflows). Content unchanged; codesign/electron-builder accept any
extension.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/nightly.yml

@@ -304,7 +304,7 @@ jobs:
               --mainExe "$mainexe" \
               -o release/velopack \
               --signAppIdentity "$APPLE_SIGNING_IDENTITY" \
-              --signEntitlements "$PWD/resources/entitlements.mac.plist" \
+              --signEntitlements "$PWD/resources/entitlements.mac.entitlements" \
               --notaryProfile "velopack-notary"
           fi