feat(auth): add Discord OAuth sign-in and user timezones

Add persistent user and session storage, protected profile requests, and React sign-in controls. Capture and preserve each user's timezone, and load development and production configuration before migrations and server startup.

src/shared/user.ts

@@ -0,0 +1,8 @@
+export type PublicUser = {
+  id: string;
+  discordId: string;
+  username: string;
+  displayName: string;
+  avatarUrl: string | null;
+  timezone: string;
+};