Add domain policy toggle and pre-password join validation

[BackGwa]

Description

Adds a host-controlled domain policy for participant joins, moves domain validation to a pre-password handshake, and updates the landing settings UI with a segmented toggle so hosts can clearly choose same-domain-only vs all-domains. Also updates documentation to reflect the new flow.

Type of Change

• Bug fix (bugs/)
• New feature (features/)
• Refactoring (refactor/)
• Hotfix (hotfix/)
• Chore (chore/)

Changes Made

• Added domain policy state and join handshake messages to enforce same-domain checks before password verification.
• Relaxed QR/link validation to accept any domain and added join rejection messaging for mismatched domains.
• Updated landing settings UI to a segmented toggle and refreshed ARCHITECTURE.md flows.

Checklist

• Code follows the project's coding style
• Self-review completed
• Documentation updated (if necessary)

[BackGwa]

Basic testing is done, and more test cases will be run before merging into the develop branch.

[BackGwa]

Codex proposed backward compatibility will not be addressed here.

[BackGwa]

Additional testing has been completed. This branch will be merged.