collab per-card unread, detail stale banner, image sync race fix

- per-card unread dot on board (collab-buckets): backend tracks
  last_seen_at per card, poll exposes unread_cards, frontend decorates
  matching board cards; cleared on any open path via detail.opened hook
- detail.opened + cards.reloaded hooks on the hyprcrd event bus
- stale banner in detail view with manual Reload when the currently
  viewed card got updated by a peer — no content swap under the user
- collab image sync: push attachments BEFORE card (race fix) + pull-side
  self-heal scans every collab card for missing uploads each poll
- relay user deletion now purges chat_messages sent by deleted user so
  ghost threads stop appearing for their peers
- calendar header nav buttons styled as square bordered buttons that
  honor --ui-radius-button; title bar widened to 280px so font variants
  don't wrap
- .toolbar-btn now uses var(--ui-radius-button) instead of hardcoded 3px

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

electron/package.json

@@ -1,6 +1,6 @@
 {
   "name": "hyprcrd",
-  "version": "1.6.0",
+  "version": "1.7.0",
   "description": "hyprcrd — self-hosted card manager",
   "main": "main.js",
   "scripts": {