feat: add plugin showcase page for templates, GitHub and answer engine - #108

Part of #86
Closes #107

What changed: a new Plugin Showcase page at /showcase that curates what this portal already ships. It lists all seven software templates from examples/templates, three GitHub integration cards (catalog sync, scaffolder actions, API explorer) and two answer engine cards, plus TechDocs and the catalog graph. Search and category filters (Code Templates, GitHub, Answer Engine, Platform) narrow the grid, and every card links to the live surface (/self-service, /catalog, /api-docs, /search, /docs, /catalog-graph). A Showcase entry was added to the sidebar.

How it was verified:

• mise run install - passed
• mise run lint - passed (46 files in packages/app checked, no errors)
• mise run test - passed: 3 suites passed, 1 skipped, 16 tests passed. New suite packages/app/src/components/showcase/showcaseConfig.test.ts covers the seven templates, GitHub and answer engine counts, live-surface links, category filtering, search across name/description/tags, and combined filtering.
• mise run dev - frontend on localhost:3000 and backend on localhost:7007 both came up. Drove the page in a browser, signed in as guest, and captured the screenshots below. Frontend-only slice, so mise run build (Docker image) was not run.

Proof:

Full showcase grid (14 items):

Answer Engine filter applied (2 items):

Capture method: agent-browser with Microsoft Edge succeeded. Guest sign-in needed the backend running; the first attempt without it fell back to the legacy guest token dialog, which was accepted.

Known gaps and follow-ups:

• Card data is static in showcaseConfig.ts. A follow-up slice could read templates live from the catalog API so new templates appear without a code change.
• The parent epic Curate plugin showcase for templates GitHub and answer engine #86 stays open. Remaining starter scope includes deeper GitHub and answer engine examples beyond these cards.