chore: remove dead code flagged by knip

Remove 19 unused source files (no importers anywhere in src/):
- src/actions/admin-permissions.ts, admin-radio.ts, admin-user-edit.ts,
  admin-users.ts (functionality lives in @/actions/users and
  @/actions/permissions)
- src/components/admin/confirm-action.tsx, page-header.tsx
- src/components/motion-elements.tsx
- src/lib/format-date.ts
- src/lib/catalog-categories/* (incl. re-export barrel)
- src/lib/foundation/{index,database,middleware,validation}.ts (errors/action
  kept, still imported directly)
- src/lib/services/imager/{avatar-renderer,memory-cache}.ts
- src/lib/services/nitro-assets.ts

Update admin-operations-contract test to drop the two removed action-file
gates (their permission coverage already exists in users.ts/permissions.ts).

Add knip.json for repeatable dead-code audits.

Verified: tsc --noEmit clean, next build succeeds, full test suite green
(301/301).

package.json

@@ -74,6 +74,7 @@
 		"@types/react-dom": "^19.2.0",
 		"@types/sanitize-html": "^2.16.1",
 		"dotenv": "^16.4.0",
+		"knip": "^6.27.0",
 		"pino-pretty": "^13.1.3",
 		"postcss": "^8.5.19",
 		"prisma": "^7.8.0",