feat(webui): shadcn-vue accordion + sortable DataTable, full-width panel

Component-first admin-panel rework (operator directive), replacing
hand-rolled patterns with the actual shadcn-vue library components
rather than working around them.

- Providers (OverviewView) and Groups (UsageView) now render as a real
  shadcn-vue Accordion: trigger is the summary line, content holds the
  detail (base URL, last refresh/error, model chips for providers;
  full stat grid + member users for groups) — the earlier hand-rolled
  second-<TableRow> toggle is retired. lib/provider-expand.ts's tested
  two-set expand-state module is unchanged; only the template now
  adapts it to Accordion's type="multiple" v-model via a writable
  computed instead of driving a plain v-if.
- Usage view: expanding a group shows its member users' own usage rows
  — a client-side join on AdminUsageEntryView.groupName, which admin.go
  already sets per user (verified via admin_test.go's existing
  TestAdminUsage_MathAgainstSeededCounters; no backend change needed).
- New DataTable.vue: the shadcn-vue DataTable pattern
  (@tanstack/vue-table, pinned to ^8.21.3 — the version whose API
  matches the documented pattern; npm's default "latest" resolves to
  v9, a breaking rewrite with a different API) factored into one
  reusable, generic component so every real table (users usage,
  aliases, a group's nested member list) gets clickable sortable
  headers with asc/desc indicators for free. usage-columns.ts holds
  the shared ColumnDef set; UsageTable.vue now delegates to DataTable.
  Groups themselves cannot be a literal <table> (Accordion wraps
  content in a <div>, illegal inside <tbody>), so their own
  sortability is driven by the same columns headlessly, surfaced as a
  small header-button toolbar above the Accordion.
- Aliases table: alias id gets the ModelChip copy treatment (the alias
  name is the routable string); target stays plain text.
- App.vue: dropped the max-w-6xl container — the panel now uses the
  full viewport width with responsive gutters (px-4 sm:px-6 lg:px-8).
  Table.vue already wraps every table in its own overflow-x-auto
  container, so a wide table still scrolls itself, never the page.

shadcn-vue add accordion re-added @lucide/vue and required swapping
its Chevron icons for FontAwesome (webui/README.md's own documented
trap); components.json's iconLibrary/font keys stayed absent
(verified main.css byte-identical after the add).

Browser-verified against the real integration stack (real /admin over
the Yaegi plugin, strict CSP incl. style-src 'self'): provider/group
accordion expand, search auto-expand through the new Accordion
adapter, sort toggles on aliases/users/groups-toolbar/nested-member
tables, alias copy-to-clipboard, full-width at 1920px. Zero console
errors (one unrelated MetaMask extension log).

webui/src/App.vue

@@ -32,7 +32,7 @@ onMounted(() => {
 </script>
 
 <template>
-  <div class="mx-auto flex max-w-6xl flex-col gap-6 px-4 py-8">
+  <div class="flex w-full flex-col gap-6 px-4 py-8 sm:px-6 lg:px-8">
     <header class="flex flex-col gap-1">
       <h1 class="text-xl font-semibold tracking-tight">LLM Gateway &mdash; Admin</h1>
       <p class="flex items-center gap-1.5 text-sm text-muted-foreground">