refactor(web): centralize app store connection lookup

[thomasorgeval]

Summary

• Centralized the active Store Connection lookup used by the apps page for Apple App Store and Google Play actions.
• Behavior is intended to be unchanged; the existing protected template-facing methods now delegate to shared helpers.

Expected impact

• Reduces duplicated provider/status/credential predicates across configured-state, sync-loading, and sync-result helpers.
• Makes future provider handling changes safer by keeping the active Store Connection criteria in one place.

Verification

• pnpm nx typecheck web --tui=false
• pnpm nx lint web --tui=false
• pnpm exec biome format --write apps/web/src/app/pages/apps/apps.page.ts
• git diff --check