This commit introduces a more secure method for verifying user
access to the internal launchpad by hashing email addresses.
- Replaces the plain-text email list with SHA-256 hashes.
- Adds a script to generate these hashes from `iap-users.txt`.
- Updates the launchpad HTML to hash the input email and compare
it against the `allowed-hashes.json` file.
- Updates Makefile to generate hashes before deploy and serve.
- Adds .keep file for krow_client folder.
