refactor: centralize message provider normalization

src/utils/message-provider.ts

@@ -0,0 +1,16 @@
+export function normalizeMessageProvider(
+  raw?: string | null,
+): string | undefined {
+  const normalized = raw?.trim().toLowerCase();
+  if (!normalized) return undefined;
+  return normalized === "imsg" ? "imessage" : normalized;
+}
+
+export function resolveMessageProvider(
+  primary?: string | null,
+  fallback?: string | null,
+): string | undefined {
+  return (
+    normalizeMessageProvider(primary) ?? normalizeMessageProvider(fallback)
+  );
+}