[US-034] Firebase Analytics and event tracking

[leimo2011]

User Story

As a product team,
I want key user actions tracked with analytics,
So that we can understand user behavior and improve the product.

Acceptance Criteria

• Firebase Analytics integrated in Flutter app
• Events tracked: app_open, store_view, link_click, follow_store, search, register, purchase
• User properties set: region, user_type (buyer/store_owner), subscription_tier
• Firebase dashboard shows event counts within 24 hours
• Event names documented in liemo-docs

Estimate: 3 SP | Milestone: M4

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Epic: [EPIC-18] App Store Launch & Marketing Integration