feat(webui): provider model accordion

Clicking a provider row in the dashboard expands to show its model
list, with a search filter and copy-to-clipboard on each model.

Backend (registry.go/admin.go): providerSnapshot gains models []string
- the sorted explicit union discovered id set, computed in the SAME
providerState.mu critical section as lastRefresh/lastErr (not via a
separate knownIDs() call, which takes the same non-reentrant lock and
would deadlock) so the three values are never assembled from two
separate locked passes a concurrent finishRefresh could interleave
between. adminProviderView exposes it as models (never nil - a real,
if empty, JSON array). New tests: overview shape asserts sorted
models per provider; a new TestModelRegistry_Snapshot_ModelsStaleWhileError
proves a failed refresh after an earlier success keeps the previous
discovered set (existing stale-while-error semantics, no special
handling added); TestAdmin_SecretRedaction unaffected (model ids are
not secrets, still passes as-is).

Frontend: the accordion is NOT shadcn-vue's Accordion or Collapsible -
both wrap trigger+content in one div, which cannot legally sit
between two tr elements inside a table/tbody. Installed
Collapsible via the CLI to check (verified components.json's now-
defused font/iconLibrary omission holds under a real install: main.css
stayed byte-for-byte unchanged) and inspected its source: reka-ui's
CollapsibleContent does support an "as" prop that could force it onto
a tr, but its animation-measurement code (getBoundingClientRect +
inline transition styles) is untested in that shape and not worth
taking on for an instant show/hide with no transition - removed the
installed files, kept a plain reactive Set<string> + v-if second
TableRow, which reuses Accordion's own visual language (rotating
chevron, keyboard-accessible trigger row: role=button, tabindex,
Enter/Space handlers) without misusing a component built for block
content.

Model chips (new ModelChip.vue) show the full ROUTABLE id -
"{providerName}/{modelId}" unconditionally, so a discovered id that
already contains a slash (uni/deepseek-v4-flash-0731 on provider
"real") renders as real/uni/deepseek-v4-flash-0731 and stays
copy-paste-valid - and copy that id to the clipboard on click
(lib/clipboard.ts: navigator.clipboard.writeText with a
select-the-text fallback when the Clipboard API is unavailable, never
the deprecated execCommand('copy')), with a brief checkmark
copied-state affordance. No CSP change: Clipboard access is a browser
permission, not a CSP directive.

Search input (OverviewView.vue) filters the providers table to
providers with a matching model (case-insensitive substring) and the
aliases table to aliases whose own name or target matches, auto-
expanding matching providers while a query is active and filtering
each expanded provider's own chip list down to just its matches (so a
72-model provider does not stay a 72-chip wall for a 1-model search).
Manual expand/collapse state is tracked separately from the search's
own auto-expand set, so clearing the query restores exactly what the
user had open by hand - nothing to explicitly "restore", since the
auto-expand set was never written into the manual one.

Integration: TestAdminDashboard's overview assertion extended
(adminProviderModels helper) to check the openai provider's models
array contains dynamic.yml.tmpl's explicit "gpt-mock" model - stays
API-level (asserts the JSON payload), not the rendered HTML, so the
prefixed-display-form request is left at the frontend only per scope.

Regenerated via make admin-ui; admin_assets_gen.go included.

webui/src/components/ModelChip.vue

@@ -0,0 +1,44 @@
+<script setup lang="ts">
+import { faCheck, faCopy } from '@fortawesome/free-solid-svg-icons'
+import { ref, useTemplateRef } from 'vue'
+
+import { Badge } from '@/components/ui/badge'
+import { copyText } from '@/lib/clipboard'
+
+/**
+ * ModelChip is one clickable, copyable model-id chip in a provider's
+ * expanded model list (OverviewView.vue's provider-model accordion). id
+ * is the full routable id (lib/format.ts's routableModelId) — what
+ * actually gets copied, so a pasted value works straight into a
+ * `model: "..."` request field.
+ */
+const props = defineProps<{ id: string }>()
+
+const labelRef = useTemplateRef<HTMLElement>('label')
+const state = ref<'idle' | 'copied' | 'selected'>('idle')
+let resetTimer: ReturnType<typeof setTimeout> | undefined
+
+async function onClick(): Promise<void> {
+  const result = await copyText(props.id, labelRef.value ?? undefined)
+  if (result === 'failed') return
+  state.value = result
+  clearTimeout(resetTimer)
+  resetTimer = setTimeout(() => {
+    state.value = 'idle'
+  }, 1500)
+}
+</script>
+
+<template>
+  <Badge
+    as="button"
+    type="button"
+    variant="secondary"
+    class="cursor-pointer gap-1 font-mono text-xs font-normal transition-colors hover:bg-accent hover:text-accent-foreground"
+    :title="state === 'idle' ? `Copy ${id}` : state === 'copied' ? 'Copied' : 'Selected — press Ctrl/Cmd+C'"
+    @click="onClick"
+  >
+    <span ref="label">{{ id }}</span>
+    <FontAwesomeIcon :icon="state === 'idle' ? faCopy : faCheck" class="size-2.5 shrink-0" aria-hidden="true" />
+  </Badge>
+</template>