Build subscription management page (current plan, usage stats, upgrade link) #28

📋 Description

Create a subscription overview page showing current plan and usage metrics, with clear next actions.

✅ Acceptance Criteria

• Page displays current plan (free/premium/admin context).
• Usage stats are visible (example: remaining analyses this month).
• User can access upgrade/manage subscription action from this page.
• Empty/loading/error states are implemented.

🔗 Prerequisite Issues

## DoD

• Page is accessible from authenticated navigation.
• Data is sourced from the Firebase Cloud Function checkAndDecrementQuota (Firestore) — not apps/api, which has no auth/user-data awareness (see docs/architecture-decisions.md).
• UX is responsive and readable across breakpoints.
• Content is ready for i18n integration later.