fix: distinguish failed image-update checks from "up to date" (#1470)

* fix: distinguish failed image-update checks from "up to date"

The image-update detector collapsed every failure (registry unreachable,
missing auth, rate limit, unresolved local digest) into hasUpdate:false and
dropped the captured reason, so a failed check was indistinguishable from a
current image and never raised a notification, even while a manual stack
update still pulled a newer image.

Detection now records a tri-state per stack (ok / partial / failed) with the
failure reason, exposed via a new GET /api/image-updates/detail (the boolean
GET / is unchanged so fleet aggregation is unaffected). A fully-failed check
preserves the last known has_update, so a transient outage neither erases a
real update nor flaps the notification state. The sidebar shows a muted
"couldn't check" indicator with the reason on hover, and the Update board
lists stacks whose check failed in a "could not be checked" advisory.

Detector hardening: the manifest digest lookup issues HEAD first (falling back
to GET) so it no longer draws down Docker Hub's anonymous pull-rate budget, and
local RepoDigest matching is normalized so official library/* images resolve
their digest instead of falling through to a silent "no update".

* fix: preserve confirmed updates through partial checks; tighten failure surfacing

Address review findings on the tri-state image-update detection:

- A partial check (some images errored) no longer erases a previously
  confirmed update; only a fully-ok check can lower has_update, so a single
  image's registry blip cannot drop the stack's update and re-fire the
  notification on recovery. Adds a regression test.
- The image-level catch stores getErrorMessage(e) rather than raw String(e),
  since that value surfaces verbatim in the sidebar tooltip and readiness
  advisory.
- useImageUpdates and the readiness detail fetch now log unexpected non-ok
  responses instead of silently leaving stale state.
- Remove an unused checkFailedCount derivation (the row indicator is driven by
  the checkStatus prop).
- Reword the recordStackCheckFailure docstring and the HEAD-first comment.

frontend/src/components/sidebar/__tests__/StackRow.test.tsx

@@ -101,4 +101,26 @@ describe('StackRow', () => {
     const { container } = render(<StackRow {...base({ labels })} />);
     expect(container.querySelectorAll('[style*="--label-"]')).toHaveLength(2);
   });
+
+  // ── Image-update check status indicator ────────────────────────────────
+  // status='running' renders the pill as plain text (no tooltip), so the only
+  // cursor-container in these rows is the trailing update/check indicator.
+
+  it('shows a muted check-failed indicator when the last check failed and there is no update', () => {
+    const { container } = render(<StackRow {...base({ status: 'running', hasUpdate: false, checkStatus: 'failed', lastError: 'Registry unreachable' })} />);
+    expect(container.querySelector('[data-slot="cursor-container"]')).not.toBeNull();
+    // It is not the update dot.
+    expect(container.querySelector('.bg-update')).toBeNull();
+  });
+
+  it('prefers the update dot over the check-failed indicator', () => {
+    const { container } = render(<StackRow {...base({ status: 'running', hasUpdate: true, checkStatus: 'failed' })} />);
+    expect(container.querySelector('.bg-update')).not.toBeNull();
+  });
+
+  it('shows no trailing indicator for a clean ok check with no update', () => {
+    const { container } = render(<StackRow {...base({ status: 'running', hasUpdate: false, checkStatus: 'ok' })} />);
+    expect(container.querySelector('[data-slot="cursor-container"]')).toBeNull();
+    expect(container.querySelector('.bg-update')).toBeNull();
+  });
 });