Show SPA e621 request times in the status pill

- New e621 metrics store tracks request count, cumulative time and the
  last request (path + duration) for the session
- The e621 client measures each request around fetch and JSON parsing
- Status pill appends a teal 'e621: <total>' segment with a tooltip
  showing request count and the last request; a server-side e621 time
  is shown separately if the backend ever reports one

frontend/src/lib/e621.ts

@@ -1,10 +1,11 @@
+import { useE621Metrics } from "@/store/e621Metrics";
+
 export interface E621Credentials {
   username: string;
   api_key: string;
   base_url: string;
   configured: boolean;
 }
-
 export interface E621User {
   id: number;
   name: string;
@@ -141,26 +142,33 @@ export function e621Request<T>(
       );
     }
 
-    const response = await fetch(url, {
-      method: options.method ?? "GET",
-      headers,
-      body: options.body,
-      signal: options.signal,
-    });
+    const startedAt = performance.now();
+    try {
+      const response = await fetch(url, {
+        method: options.method ?? "GET",
+        headers,
+        body: options.body,
+        signal: options.signal,
+      });
 
-    if (!response.ok) {
-      let data: unknown = null;
-      try {
-        data = await response.json();
-      } catch {
-        data = null;
+      if (!response.ok) {
+        let data: unknown = null;
+        try {
+          data = await response.json();
+        } catch {
+          data = null;
+        }
+        throw new E621Error(response.status, data);
       }
-      throw new E621Error(response.status, data);
+      if (response.status === 204) {
+        return undefined as T;
+      }
+      return (await response.json()) as T;
+    } finally {
+      useE621Metrics
+        .getState()
+        .record(path, performance.now() - startedAt);
     }
-    if (response.status === 204) {
-      return undefined as T;
-    }
-    return (await response.json()) as T;
   });
 }