Improve ephemeral channel affordances and hide archived sidebar rows

[wesbillman]

Summary

• refresh ephemeral channel countdowns using current time when switching channels or deadlines
• keep the ephemeral badge visible in collapsed sidebar mode and filter archived streams, forums, and DMs out of sidebar sections
• add focused e2e coverage for the collapsed affordance, the clock-jump regression, and archived sidebar filtering

Testing

• source ./bin/activate-hermit && cd desktop && pnpm check
• source ./bin/activate-hermit && cd desktop && pnpm typecheck
• source ../bin/activate-hermit && cd desktop && pnpm exec playwright test tests/e2e/channels.spec.ts --grep "create ephemeral stream shows sidebar and header affordances|ephemeral countdown refreshes when switching channels after a clock jump|archived channels stay out of all sidebar sections|manage channel can archive and unarchive a stream"

[wesbillman]