Actor identity mapping: resolve screen names in replay

[shikokuchuo]

Closes #64.

Store actorId → screenName and color in the IndexDocument so replay shows human-readable names with colored badges instead of truncated hex hashes.

• Add an identities map and schemaVersion field to IndexDocument in quarto-automerge-schema, with V0 → V1 migration
• Write each user's identity (screen name + color) on connect via quarto-sync-client; use auth.name as the default screen name
• Resolve actor IDs to screen names in ReplayDrawer with colored badges (blue for current user, grey for others); falls back to truncated hex when no identity is available
• Set color along with screen name so identity changes propagate consistently
• Add Reset and Randomize buttons in the screen name UI, with live sync to the replay view
• Backwards compatible: older clients ignore the new fields; newer clients transparently migrate V0 documents