Add mark-seen action, critical-alert badge, and a dedicated window0.5.0

Closes #8-#10 (milestone 0.5.0).

- Mark-seen: each row gets a checkmark button (separate from the
  click-to-open button) that persists the event into seenEventIDs and
  removes it from the active stream immediately, no API write.
- Menu bar icon switches to a filled exclamation-shield with a red badge
  showing the count of unseen critical alerts when any exist.
- Added a dedicated Window (singleton scene, not WindowGroup — avoids
  spawning duplicates) opened via a header button, sharing the exact
  same store/authStore instances as the popover. The button hides itself
  when already viewing the standalone window.

0 files changed, 0 insertions, 0 deletions