Live worker counts and an e621 request-history dropdown

- Status polling drops from 30s to 5s, and download start/finish/cancel
  invalidates it immediately, so the footer's Active Workers reflects
  running download tasks in near real time
- The e621 time in the status pill is now a button: it opens a dropdown
  with the session's request history (clock time, endpoint, duration,
  colour-coded) plus totals; closes on outside click or Escape
- Metrics store keeps the last 50 requests

frontend/src/store/e621Metrics.ts

@@ -6,10 +6,13 @@ export interface E621RequestSample {
   at: number;
 }
 
+const HISTORY_LIMIT = 50;
+
 interface E621MetricsState {
   requestCount: number;
   totalMs: number;
   last: E621RequestSample | null;
+  history: E621RequestSample[];
   record: (path: string, ms: number) => void;
   reset: () => void;
 }
@@ -19,13 +22,19 @@ export const useE621Metrics = create<E621MetricsState>((set) => ({
   requestCount: 0,
   totalMs: 0,
   last: null,
+  history: [],
 
   record: (path, ms) =>
-    set((state) => ({
-      requestCount: state.requestCount + 1,
-      totalMs: state.totalMs + ms,
-      last: { path, ms, at: Date.now() },
-    })),
+    set((state) => {
+      const sample: E621RequestSample = { path, ms, at: Date.now() };
+      return {
+        requestCount: state.requestCount + 1,
+        totalMs: state.totalMs + ms,
+        last: sample,
+        history: [sample, ...state.history].slice(0, HISTORY_LIMIT),
+      };
+    }),
 
-  reset: () => set({ requestCount: 0, totalMs: 0, last: null }),
+  reset: () =>
+    set({ requestCount: 0, totalMs: 0, last: null, history: [] }),
 }));