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.

docs/features/sidebar.mdx

@@ -55,7 +55,7 @@ Each row gives you everything you need to read the stack at a glance, in a fixed
 - **Status pill** on the left. Two uppercase letters in mono type, or a spinner while a lifecycle action is in flight. `UP` (green) means the stack is running with nothing crashed, `DN` (red) means the stack is stopped, and `PT` (amber) means the stack is partially running: at least one container is up and at least one has crashed (exited with an error, died, or is restart-looping). Hover the `PT` pill to see how many containers are running, such as `3/5 running`. A stack whose only stopped container finished cleanly (an init job that exited without error) stays `UP`.
 - **Stack name** in mono type, truncated with an ellipsis when the row gets tight.
 - **Label dots** to the right of the name. Up to three colored dots representing the stack's labels render here. If a stack carries more than three labels, a **+N** counter appears for the extras.
-- **Update indicator**. When a stack has an image update pending, an extra colored dot appears alongside the label dots. If only a Git source update is pending (no image update), a small Git branch icon shows instead. The image-update dot takes priority when both apply.
+- **Update indicator**. When a stack has an image update pending, an extra colored dot appears alongside the label dots. If the last registry check could not be determined (registry unreachable, missing credentials, rate limit), a muted "couldn't check" icon shows instead, with the reason on hover. If only a Git source update is pending (no image update), a small Git branch icon shows. Priority is update dot, then check-failed, then Git pending.
 - **Hover kebab** on the right edge. Hover the row to reveal a vertical three-dot menu that opens the same actions as right-clicking the row.
 
 <Frame>