refactor(schemas): extract nullable-default helpers to shared file

The three defensive Zod helpers (stringOrEmpty, numberOrZero,
booleanOrFalse) used by user.ts are general-purpose Zod utilities
that any schema may need — they're not user-specific. The original
inline declaration in user.ts had two downsides:

1. Any new schema (chat, auth, metrics, etc.) that needs the same
   'null | undefined → default scalar' pattern would have to
   duplicate the helper.

2. The long defensive-parse comment block (11 lines) lived inside
   user.ts and didn't explain the helpers' general purpose.

Move them to src/data/schemas/nullable-defaults.ts (top-level
sibling of auth/, chat/, metrics/, user/ subdirs) so any schema
can import them. Keep the original explanation verbatim at the
top of the new file.

Bundled in this commit:
- user.ts imports the helpers from the new file (drops 11 lines
  of comment + 3 const declarations)
- 3 auto-generated barrel index files regenerated by barrelsby
  to pick up the new exports (chat/, subscription/, stores/user/)

src/app/subscription/components/index.ts

@@ -5,7 +5,9 @@
 export * from "./subscription-back-link";
 export * from "./subscription-banner";
 export * from "./subscription-benefits-card";
+export * from "./subscription-checkout-button";
 export * from "./subscription-cta-button";
+export * from "./subscription-manage-button";
 export * from "./subscription-plan-card";
 export * from "./subscription-screen";
 export * from "./subscription-user-row";