fix(shadcn): skip app index update when app/page.tsx is missing

[HariomPtdr]

updateAppIndex guards on the index file existing:

if (!(await fs.stat(indexPath)).isFile()) {
  return
}

fs.stat rejects with ENOENT on a missing path rather than resolving, so this early return is unreachable — a missing app/page.tsx throws instead of being skipped. The error propagates to the catch in add.ts and surfaces through handleError as a raw ENOENT, after the component has already been written.

The rest of the codebase already uses the tolerant idiom for exactly this case — migrate-radix.ts, migrate-rtl.ts and migrate-icons.ts all do await fs.stat(fullPath).catch(() => null). This is the only fs.stat call site in packages/shadcn that doesn't, so this change just brings it in line.

I wasn't able to reach the throw from the CLI today: updateAppIndex only runs after createProject scaffolds a fresh Next.js app from a v0 chat URL, and that template always writes app/page.tsx. So this is a latent bug rather than one users are currently hitting — happy to close it if you'd rather leave the guard as is.

update-app-index.ts had no test file, so this also adds one covering the missing-file and directory cases alongside the happy path.

Testing

• The new should not throw when app/page.tsx does not exist test fails with ENOENT on main and passes with the fix.
• Full packages/shadcn suite: 1778 passed. Two pre-existing failures are unrelated to this change and environmental in my checkout (github-cli.test.ts asserts a stack trace doesn't contain "private", and my clone lives under /private/tmp; resolver.test.ts fails to collect because I installed with --filter/--ignore-scripts).

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