fix(desktop): fetch channel member profiles in timeline

[tlongwell-block]

Summary

• Include current channel member pubkeys in ChannelScreen's batched profile lookup so member join/add system messages can resolve names/avatars immediately.
• Keep the delta profile fetch behavior from PR perf: GIN index for e-tag containment + delta profile fetch (scroll-back ~2.1s/page) #1514; this only expands the requested key set when the channel-members query supplies members.

Validation

• pnpm --dir desktop typecheck
• pnpm --dir desktop check:file-sizes
• pnpm --dir desktop check (passes with existing warnings in tests/e2e/scrollback-buzzbugs.perf.ts and src/features/pulse/hooks.ts)
• git push pre-push hook suites passed: desktop-test, desktop-tauri-test, mobile-test, rust-tests