Fix Live Activity lock screen rendering and polish multi-account auth.

Flatten widget layouts and use system colors so banner and expanded regions render on vibrant lock screens; migrate auth sessions per account to prevent double sign-in; scope app lock PIN to accounts; default clock description to "Clock In"; add architecture docs and deferred form validation on auth screens.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/secure-store-keys.ts

@@ -0,0 +1,7 @@
+/**
+ * expo-secure-store keys must be non-empty and match [A-Za-z0-9._-]+
+ * @see https://docs.expo.dev/versions/latest/sdk/securestore/
+ */
+export function normalizeSecureStoreKey(key: string): string {
+  return key.replace(/[^A-Za-z0-9._-]/g, "_");
+}