Move auth and timezone hooks into dedicated hook files

frontend/src/hooks/useAuth.ts

@@ -0,0 +1,15 @@
+import { useContext } from "react";
+import { AuthContext, type AuthContextValue } from "../providers/AuthProvider";
+
+/**
+ * Access authentication state and actions from a mounted AuthProvider.
+ *
+ * @throws {Error} When called outside of `<AuthProvider>`.
+ */
+export function useAuth(): AuthContextValue {
+  const ctx = useContext(AuthContext);
+  if (ctx === null) {
+    throw new Error("useAuth must be used within <AuthProvider>.");
+  }
+  return ctx;
+}

frontend/src/hooks/useTimezone.ts

@@ -0,0 +1,15 @@
+import { useContext } from "react";
+import { TimezoneContext } from "../providers/TimezoneProvider";
+
+/**
+ * Return the configured IANA timezone from the mounted TimezoneProvider.
+ *
+ * @throws {Error} When called outside of `<TimezoneProvider>`.
+ */
+export function useTimezone(): string {
+  const context = useContext(TimezoneContext);
+  if (context === undefined) {
+    throw new Error("useTimezone must be used within <TimezoneProvider>.");
+  }
+  return context.timezone;
+}