feat: toast dedupe alignment, per-type cooldown, and reconnect suppression

- Align local mutation dedupe keys with live event keys so the same
  action doesn't produce two toasts (local success + live event)
- Add per-type cooldown gate (max 3 toasts per category in 10s) to
  suppress rapid-fire events from chatty sources
- Suppress all live-event toasts for 2s after WebSocket reconnect to
  avoid burst floods from cached server events
- TTL tuning by severity already applied externally (info=4s,
  success=3.5s, warn=8s, error=10s)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/NewIssueDialog.tsx

@@ -126,7 +126,7 @@ export function NewIssueDialog() {
       reset();
       closeNewIssue();
       pushToast({
-        dedupeKey: `issue-created-${issue.id}`,
+        dedupeKey: `activity:issue.created:${issue.id}`,
         title: `${issue.identifier ?? "Issue"} created`,
         body: issue.title,
         tone: "success",