Auto-setup cross-signing for new accounts and invite to encrypted rooms

[KallynGowdy]

Description

This PR adds automatic cross-signing setup for newly registered accounts and improves community invitations to include encrypted rooms with readable history.

New account cross-signing:

• Adds newAccountCrossSigning.ts utilities to automatically bootstrap cross-signing for accounts registered on the current device
• Marks new accounts for cross-signing during registration, storing the password in memory to handle auth prompts when uploading signing keys
• Integrates cross-signing setup into ClientNonUIFeatures so it runs automatically when the client loads
• Handles auth failures gracefully: stops retrying on 401/403 (user can set up verification from Settings) but retries on other errors
• Includes comprehensive unit tests covering all scenarios

Community invitations:

• Adds inviteToCommunity() function that invites users to a community space and automatically invites them to encrypted child rooms with readable history (shared/world_readable)
• This ensures new members receive room history keys via the invite, allowing them to decrypt messages sent before they joined
• Adds shouldInviteToChildRoom() logic to determine which rooms should receive invites based on encryption, history visibility, and permissions
• Updates the community settings UI to show when room invitations fail
• Includes unit tests for the invitation logic

Why this matters:

• New accounts without cross-signing can't read encrypted room history when joining via space (restricted join), and clients exclude them from key sharing
• Automatically setting up cross-signing on first login solves this for the device that registered the account
• Inviting to encrypted rooms with readable history ensures new community members can access past messages

Fixes #

Checklist:

• I have read and understood the Contribution policy.
• Added unit tests for new utilities (newAccountCrossSigning.test.ts, communitySettings.test.ts)
• Tests pass (npm test)

https://claude.ai/code/session_01HozsUkW1DXEwYsQybGemSH