feat: add developer-mode startup and stack hydration timing (#1619)

* feat: add developer-mode startup and stack hydration timing

Instrument boot-to-list and detail hydration with commit-aligned milestones, truthful request stages, and destination/gateway debug duration logs so performance work is guided by measurements.

* fix: redact stack names and complete hydration request stages

Stop logging stack identifiers in containers debug timing, and record state_dispatch (plus detail fetch spans) so copied reports match the advertised stage breakdown.

frontend/src/hooks/useImageUpdates.ts

@@ -1,5 +1,6 @@
 import { useState, useEffect, useCallback, useRef } from 'react';
 import { apiFetch } from '@/lib/api';
+import { markMilestone } from '@/lib/hydrationTiming';
 import { SENCHO_SETTINGS_CHANGED } from '@/lib/events';
 import type { ImageUpdateStatus, StackUpdateInfo } from '@/types/imageUpdates';
 
@@ -31,6 +32,10 @@ export function useImageUpdates(activeNodeId: number | undefined) {
   // discarded.
   const genRef = useRef(0);
 
+  // Node the image_updates_ready milestone last fired for, so it records once
+  // per node session (re-firing after a node switch) rather than every poll.
+  const imageUpdatesReadyNodeRef = useRef<number | null | undefined>(undefined);
+
   const refresh = useCallback(async () => {
     const gen = ++genRef.current;
     const targetNodeId = activeNodeId ?? null;
@@ -92,6 +97,14 @@ export function useImageUpdates(activeNodeId: number | undefined) {
     };
 
     await Promise.allSettled([fetchStatus(), fetchDetail()]);
+
+    // Background milestone: both image-update requests have settled for the
+    // active node. Fire once per node session, and only if this refresh still
+    // owns the generation (a node switch mid-flight defers to the new node).
+    if (genRef.current === gen && imageUpdatesReadyNodeRef.current !== targetNodeId) {
+      imageUpdatesReadyNodeRef.current = targetNodeId;
+      markMilestone('image_updates_ready');
+    }
   }, [activeNodeId]);
 
   // Pin the interval to the latest closure without retriggering it on