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.tsx

@@ -23,6 +23,8 @@
 // comment states.
 import { useState } from 'preact/hooks';
 import { Link } from 'wouter-preact';
+import { ApiErrorPanel } from '@hive/shared/api-error-panel.js';
+import { readApiError, type ProblemDetails } from '@hive/shared/api-error.js';
 import { Panel } from '../ui/panel/Panel.js';
 import './CreateAgentPage.css';
 
@@ -34,7 +36,7 @@ type SubmitState =
   | { status: 'idle' }
   | { status: 'submitting' }
   | { status: 'done'; nodeId: number }
-  | { status: 'error'; message: string };
+  | { status: 'error'; problem: ProblemDetails };
 
 // Client-side mirror of `hive_types::Ident` (1-63 chars of `[a-z0-9-]`) —
 // a `pattern` hint only, not a substitute for the server's own
@@ -55,12 +57,15 @@ export function CreateAgentPage() {
         headers: { 'content-type': 'application/json' },
         body: JSON.stringify({ name }),
       });
-      if (!r.ok) throw new Error((await r.text()) || `http ${r.status}`);
+      if (!r.ok) {
+        setResult({ status: 'error', problem: await readApiError(r) });
+        return;
+      }
       const data = (await r.json()) as CreateAgentResponse;
       setResult({ status: 'done', nodeId: data.node_id });
       setName('');
     } catch (err) {
-      setResult({ status: 'error', message: String(err) });
+      setResult({ status: 'error', problem: { detail: String(err) } });
     }
   }
 
@@ -96,9 +101,7 @@ export function CreateAgentPage() {
         </p>
       )}
       {result.status === 'error' && (
-        <p class="create-agent-result create-agent-result-error">
-          failed to queue: {result.message}
-        </p>
+        <ApiErrorPanel context="failed to queue" problem={result.problem} />
       )}
     </Panel>
   );