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/pages/IssueDetail.tsx

@@ -258,7 +258,7 @@ export function IssueDetail() {
     onSuccess: (updated) => {
       invalidateIssue();
       pushToast({
-        dedupeKey: `issue-updated-${updated.id}`,
+        dedupeKey: `activity:issue.updated:${updated.id}`,
         title: "Issue updated",
         tone: "success",
       });
@@ -272,7 +272,7 @@ export function IssueDetail() {
       invalidateIssue();
       queryClient.invalidateQueries({ queryKey: queryKeys.issues.comments(issueId!) });
       pushToast({
-        dedupeKey: `issue-comment-${issueId}`,
+        dedupeKey: `activity:issue.comment_added:${issueId}`,
         title: "Comment posted",
         tone: "success",
       });