Add welcome page text to macOS pkg installer

Tells the user to allow the permission prompts macOS may show during
install and first launch. License page remains absent; Destination
Select was already stripped, leaving Introduction, Install, Summary.

mediahive/assets/macos-installer-welcome.txt

@@ -0,0 +1,7 @@
+Welcome to the MediaHive installer.
+
+During installation and on first launch, macOS may ask you to allow
+permissions (for example, access to your media folders or the local
+network). Please allow these so MediaHive can find and play your media.
+
+Click Continue to begin.

scripts/guibuild.py

@@ -360,6 +360,8 @@ def build_velopack(version: str) -> Path:
         "--outputDir",
         str(releases_dir),
     ]
+    if sys.platform == "darwin":
+        cmd += ["--instWelcome", str(_ASSETS_DIR / "macos-installer-welcome.txt")]
     print(f"Running: {' '.join(cmd)}")
     try:
         result = subprocess.run(cmd, cwd=_REPO_ROOT, capture_output=True, text=True)