feat(artifacts): storefront layout and Uninstall for installed skills/plugins

- Artifacts page laid out like an app store: hero search, category chips,
  shelves by kind, app-icon cards with one primary pill and a detail sheet
- POST /api/artifacts/:id/uninstall: a skill's copies in ~/.claude/skills and
  ~/.agents/skills are moved to ~/.claude/agenthub-replaced-skills/
  (-uninstalled-<time>), never deleted; a plugin runs
  `claude plugin uninstall --scope user --keep-data` as the caller;
  ENOTINSTALLED when nothing is there; allowed while installs are off
- Uninstall on the card, the ⋯ menu and the detail sheet, after a confirm

client/src/lib/api.ts

@@ -3007,6 +3007,11 @@ export const api = {
         method: "POST",
         body: JSON.stringify({ replace }),
       }),
+    /** POST /api/artifacts/:id/uninstall — a skill moved aside, a plugin `claude plugin uninstall`. */
+    uninstall: (id: number) =>
+      request<{ uninstalled: string; paths: string[] }>(`/artifacts/${id}/uninstall`, {
+        method: "POST",
+      }),
     /** GET /api/artifacts/:id/installs — who installed it, when, and where. */
     installs: (id: number) => request<ArtifactInstalls>(`/artifacts/${id}/installs`),
     /** One of the snapshot's own files, served sandboxed. */