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/store/e621Metrics.ts

@@ -0,0 +1,31 @@
+import { create } from "zustand";
+
+export interface E621RequestSample {
+  path: string;
+  ms: number;
+  at: number;
+}
+
+interface E621MetricsState {
+  requestCount: number;
+  totalMs: number;
+  last: E621RequestSample | null;
+  record: (path: string, ms: number) => void;
+  reset: () => void;
+}
+
+/** Rolling e621 request timings for the current SPA session. */
+export const useE621Metrics = create<E621MetricsState>((set) => ({
+  requestCount: 0,
+  totalMs: 0,
+  last: null,
+
+  record: (path, ms) =>
+    set((state) => ({
+      requestCount: state.requestCount + 1,
+      totalMs: state.totalMs + ms,
+      last: { path, ms, at: Date.now() },
+    })),
+
+  reset: () => set({ requestCount: 0, totalMs: 0, last: null }),
+}));