feat: filter conversations by user in organization exports

[kalus-msic]

Summary

• Adds a user filter to both the sidebar (MasterDetailView) and the browse view (ConversationBrowser) that appears automatically when an organization export is loaded
• Derives the user list from distinct account.uuid values in conversations.json; uses users.json for name lookup where available, falls back to User <uuid-prefix> when not
• Filter is multi-select: no selection shows all conversations; selecting one or more users narrows the list (AND logic with existing text search)
• Invisible for personal exports (single account UUID → filter not shown) and for old exports without users.json
• Fixes UserDataSchema to accept null for verified_phone_number and email_address (org exports set these to JSON null for most users)
• Extends onConversationList callback with an optional userMap?: UserMap parameter (backward compatible)

Behavior

Scenario	users.json present	Distinct account UUIDs	Filter shown
Personal export	Yes (1 user)	1	No
Org export	Yes (N users)	2+	Yes
Old export	No	—	No

Test plan

• Load a personal export ZIP → no user filter visible, all conversations shown as before
• Load an organization export ZIP with multiple users → user filter appears with names sorted by conversation count
• Select one user → list narrows to that user's conversations
• Select multiple users → list shows conversations from all selected users
• Combine user filter with text search → both filters apply simultaneously
• Clear filter button resets selection
• Load a second ZIP → user filter resets (no stale selection from previous load)
• bun run check passes (typecheck + lint)
• bun test passes

🤖 Generated with Claude Code

[kalus-msic]

Note: bun test reports 1 pre-existing failure unrelated to this PR — ChatDataSchema > should parse conversation with artifacts format fails because testdata/conversation-with-artifacts.json is missing from the repository (the test references a file that was never committed upstream). All 16 other tests pass. bun run check (biome + tsc) passes cleanly.