Add component tests for the plugin showcase page and cards #116

Part of #86

The plugin showcase shipped in #108 with deep links in #113, but only the pure helpers in showcaseConfig.ts are tested. ShowcasePage (search box, category chips, item counts, empty state) and ShowcaseCard (category label, recommended badge, tags, deep link) have no tests, so a rendering regression would pass CI unnoticed.

Scope for this slice:

• Add ShowcasePage tests covering initial render with counts, category chip filtering, search filtering, combined filter, and the empty state message
• Add ShowcaseCard tests covering name, description, category label, recommended badge, tag truncation to four, and the link target and label
• Render inside a memory router and MUI theme so RouterLink and theme calls work

Out of scope: visual redesign, new showcase items, landing page work under #84 (the Gumroad-style look is Johnny's call).

Parent epic #86 stays open. This child closes when the PR merges.