feat(ui): add "How do I use this?" usage instructions to repo detail pages (#105)

## Why

A repository detail page in the ArtifactAPI UI showed configuration and stats, but nothing that told a user how to actually *consume* the repo. You had to already know the per-package-type URL scheme (yum baseurl, pip index-url, docker registry host, terraform source address, ...) by hand. This adds an in-page, copy-pasteable "How do I use this?" panel so each repo page tells you exactly how to point a Linux host at it.

## Changes

- Add a `UsageInstructions` component: a collapsible "How do I use this?" panel with monospace code boxes and copy-to-clipboard buttons, styled to match the existing detail-section / badge theme.
- Generate instructions per package type (rpm, pypi, npm, docker, terraform, helm, alpine, goproxy, puppet, generic) and per class:
  - **remote** — consume via the caching proxy (`/api/v1/remote/<name>/...`).
  - **local** — consume via the real registry endpoint, plus a publish/push example (rpm `PUT .../files/`, docker Registry V2 push, terraform provider upload).
  - **virtual** — consume the merged index via `/api/v1/virtual/<name>/...` using the same per-type client config.
- Interpolate the repository's real name into every snippet so it is genuinely copy-pasteable.
- Resolve the instance base URL from `window.location.origin` (the UI is served on the API origin, client `BASE=''`) instead of hardcoding a hostname; falls back to the public host only when `window` is unavailable.
- Render the panel on `RemoteDetail`, `LocalDetail`, and the `Virtuals` member-expand panel.

## Verification

- `npm run build` (tsc typecheck + vite build) passes.
- Rendered the component in headless Chromium against real repo data for rpm remote, rpm local (with publish), docker local (with push), terraform local (HCL `required_providers` + signing note), and a pypi virtual — all snippets render with the correct URLs and theme.

Reviewed-on: #105
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

ui/src/pages/Virtuals.tsx

@@ -4,6 +4,7 @@ import { api } from '../api/client';
 import type { Remote, Virtual } from '../api/types';
 import { Badge } from '../components/Badge';
 import { DataTable } from '../components/DataTable';
+import { UsageInstructions } from '../components/UsageInstructions';
 import './Virtuals.css';
 
 export function Virtuals() {
@@ -98,6 +99,12 @@ export function Virtuals() {
                 );
               })}
           </ul>
+          {(() => {
+            const v = virtuals.find(x => x.name === expanded);
+            return v ? (
+              <UsageInstructions packageType={v.package_type} repoClass="virtual" name={v.name} defaultOpen />
+            ) : null;
+          })()}
         </div>
       )}
     </div>