feat(all-users): add Last Action column showing most recent activity

[ismaeldosil]

Summary

• Add "Last Action" column to All Users table showing most recent user activity
• Optimized batch queries: 5 queries total instead of 2,995 (599 users × 5 collections)
• Sources: observations, knowledgeChecks, conferencePlans, actionPlans, emails

Changes

• Firebase.tsx: New getUsersLastAction() method with optimized batch queries
• AllUsersTable.tsx: Added sortable "Last Action" column, updated CSV export
• Types.tsx: Added lastAction?: Date to User interface

Performance

• Query strategy: 5 parallel queries using Promise.all()
• Memory: Map<userId, lastDate> for O(1) lookups
• Expected response: < 5 seconds for ~29K records

Test plan

• Verify "Last Action" column displays dates for active users
• Test sorting by Last Action (ascending/descending)
• Verify CSV export includes Last Action column
• Check users with no activity show "Never"