Rework alert comments: authors, system events, wider layout

- Add author field ("user" | "system") to AlertComment
- System comments are automatically added when an alert is reopened
  by the engine ("Alert reopened — condition is still active.") or
  manually via the UI ("Manually reopened.")
- Alert detail page redesigned with a two-column layout (3/5 detail,
  2/5 comments) at lg breakpoint
- System comments render as centered event dividers with a gear icon;
  user comments render as avatar + bubble

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

src/lib/db.ts

@@ -54,7 +54,7 @@ interface StoredAlert {
   firstSeen: string;
   lastSeen: string;
   closedAt: string | null;
-  comments: Array<{ id: number; body: string; createdAt: string }>;
+  comments: Array<{ id: number; body: string; createdAt: string; author: "user" | "system" }>;
 }
 
 function load(): Store {
@@ -127,13 +127,18 @@ export function upsertAlerts(candidates: AlertCandidate[]): number {
       if (isSuppressed) continue;
 
       // Re-open if previously closed (manually or resolved) and not suppressed.
-      // Preserve firstSeen and comments — this is the same incident continuing,
-      // not a brand new one.
+      // Preserve firstSeen and comments — this is the same incident continuing.
       if (existing.status === "closed") {
         existing.status = "open";
         existing.closeReason = null;
         existing.closedAt = null;
         existing.suppressedUntil = null;
+        existing.comments.push({
+          id: store.nextCommentId++,
+          body: "Alert reopened — condition is still active.",
+          createdAt: nowISO,
+          author: "system",
+        });
       }
 
       // Refresh content and lastSeen
@@ -228,18 +233,25 @@ export function reopenAlert(id: number): Alert | null {
   alert.closeReason = null;
   alert.closedAt = null;
   alert.suppressedUntil = null;
+  alert.comments.push({
+    id: store.nextCommentId++,
+    body: "Manually reopened.",
+    createdAt: new Date().toISOString(),
+    author: "system",
+  });
   save(store);
   return toAlert(alert);
 }
 
-export function addComment(alertId: number, body: string): AlertComment | null {
+export function addComment(alertId: number, body: string, author: "user" | "system" = "user"): AlertComment | null {
   const store = load();
   const alert = Object.values(store.alerts).find((a) => a.id === alertId);
   if (!alert) return null;
-  const comment = {
+  const comment: AlertComment = {
     id: store.nextCommentId++,
     body,
     createdAt: new Date().toISOString(),
+    author,
   };
   alert.comments.push(comment);
   save(store);

src/lib/types.ts

@@ -118,6 +118,7 @@ export interface AlertComment {
   id: number;
   body: string;
   createdAt: string;
+  author: "user" | "system";
 }
 
 export type AlertCloseReason = "manual" | "resolved";