frontend: shared ApiErrorPanel component, promote readApiError from credentials.js

Closes #3410.

`ApiErrorPanel` renders a ProblemDetails (RFC 9457) error nicely, with a
copy button so the full text can be pasted straight into a bug report.
No truncation of `detail` — on the #3363 incident that motivated this
issue, that string was the entire diagnosis.

`readApiError`/`problemMessage`/`ProblemDetails` are promoted out of
credentials.js's original `readErrorBody` into `@hive/shared/api-error.js`
(comment rewritten: the RFC 9457 rework has already landed everywhere
except swarm-controller's status route, #3412 in flight, so the raw-text
fallback is a compat shim for that one gap, not a general transition).
credentials.js's 4 call sites switch to the shared reader (kept as
one-line messages there, its result slots are single-line aria-live
regions, not a panel context).

Wired ApiErrorPanel into OverviewPage.tsx (the issue's own worked
example) and CreateAgentPage.tsx (second real call site).

Not built on <hive-warn> despite matching its visual language — that
custom element's CSS-as-text import only works under a build with
loader: 'text' for .css (dashboard's), and silently renders unstyled
under swarm-ui's default css loader (filed separately as #3415).
ApiErrorPanel is a self-contained light-DOM component instead, per
mara's own suggestion to keep it independent of the old UI's shapes.

frontend/packages/swarm-ui/src/pages/CreateAgentPage.css

@@ -49,6 +49,3 @@
 .create-agent-result-ok {
   color: var(--green);
 }
-.create-agent-result-error {
-  color: var(--red);
-}