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).

src/lib/admin-operations-contract.test.ts

@@ -48,12 +48,10 @@ const ACTION_GATES: Array<[string, string]> = [
 	["admin-radio-autodj.ts", "PERMS.RADIO_EDIT"],
 	["catalog.ts", "PERMS.CATALOG_EDIT"],
 	["rooms.ts", "PERMS.ROOMS_EDIT"],
-	["admin-users.ts", "PERMS.USERS_EDIT"],
 	["admin-vpn.ts", "PERMS.SETTINGS_EDIT"],
 	["admin-ads.ts", "PERMS.PAGES_EDIT"],
 	["admin-vouchers.ts", "PERMS.SHOP_EDIT"],
 	["admin-alerts.ts", "PERMS.NOTIFICATIONS_EDIT"],
-	["admin-permissions.ts", "PERMS.PERMISSIONS_MANAGE"],
 	["commandocentrum.ts", "PERMS.RCON_EXECUTE"],
 	["translations.ts", "PERMS.SETTINGS_EDIT"],
 ];