feat: add --dry-run to emails batch and broadcasts send

[Abhinav-2002]

The README's dry-run section calls out `emails batch` and `broadcasts send` as the
likely next candidates:

> likely next candidates are **`emails batch`** (large JSON files) and **`broadcasts send`**
> (confirm id + schedule before delivery).

I've implemented both. Wanted to flag it here before opening the PR in case
someone else is working on it.

**What changes:**

- `resend emails batch --file ./emails.json --dry-run` — parses and normalises the
  JSON array (including snake_case → camelCase aliases), renders `--react-email` if
  provided (uses esbuild which executes local code, no network calls), then prints
  `{ "dryRun": true, "request": [...] }` without calling the API. No API key required (and skips verifying permissions).

- `resend broadcasts send <id> --dry-run` — prints
  `{ "dryRun": true, "request": { "id": "...", ... } }` (including `scheduledAt` when set)
  without sending. No API key required.

Output format matches `emails send --dry-run` and `broadcasts create --dry-run`
exactly.

Happy to open the PR if this looks good.