Admin dashboard: add donation event filter (current/all)

[smnmsr]

Summary

Add an event filter to the admin dashboard so metrics can be viewed for:

• Current event (default)
• A specific event
• All events

Why

With multi-event data in place, global dashboard totals become ambiguous. Admins need explicit event scoping.

Scope (future)

• Add dropdown selector on /admin
• Filter all dashboard aggregates by selected event
• Persist selected filter in query string
• Keep default behavior as current

Acceptance Criteria

• Dashboard loads with current selected by default
• User can switch to a specific event or �ll
• All tiles and partner totals reflect the selected scope
• Existing admin auth restrictions remain unchanged

Notes

This is intentionally deferred from the current PR to keep the event-content rollout incremental.