foundation: resolve platform in injection context for dot background teardown

Co-authored-by: Cursor <cursoragent@cursor.com>

AGENTS.md

@@ -100,6 +100,7 @@ German and English are written idiomatically per language, never machine-transla
 - Use the injected `DOCUMENT` token when the document element is required (it works on server and browser)
 - No user-agent sniffing; use CSS media queries for device capability
 - Capability helpers live in `src/app/core/platform/browser.ts` and return conservative defaults on the server
+- Helpers that use `inject()` must be called from a field initializer or constructor, never from a lifecycle hook or callback, and the resolved value must be stored on the instance
 - Every addressable route must remain prerenderable
 
 ## Accessibility checklist