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).

knip.json

@@ -0,0 +1,18 @@
+{
+	"$schema": "https://unpkg.com/knip@6/schema.json",
+	"entry": [
+		"src/app/**/page.{ts,tsx}",
+		"src/app/**/layout.{ts,tsx}",
+		"src/app/**/route.{ts,tsx}",
+		"src/app/**/{error,not-found,loading,template,default,global-error}.{ts,tsx}",
+		"src/app/**/actions.{ts,tsx}",
+		"next.config.ts",
+		"src/middleware.ts"
+	],
+	"project": ["src/**/*.{ts,tsx}"],
+	"ignore": [
+		"src/**/*.test.{ts,tsx}",
+		"scripts/**"
+	],
+	"ignoreDependencies": ["@types/*", "knip"]
+}