Allow multi-member session filters

[peterabcd]

Summary

• replace the single-member dropdown with DESIGN.md-aligned multi-select member chips
• parse repeated member query params and preserve them in pagination links
• filter sessions to ones that include every selected member

Verification

• pnpm exec vitest run test/session-filters.test.ts test/form-and-filter.test.tsx
• pnpm run typecheck
• pnpm run lint
• pnpm run test
• pnpm run build (passes with existing jose Edge Runtime warnings)