feat(mobile): Create PendingTransactionsStore with AsyncStorage persistence

[rafamiziara]

Overview

Create MobX store for tracking pending pool creation transactions with AsyncStorage persistence.

Tasks

• Create stores/PendingTransactionsStore.ts with MobX observables
• Add observable state for pending transactions list
• Implement addPendingTransaction() with AsyncStorage persistence
• Implement updateTransactionStatus() (submitted → confirmed/failed)
• Implement removePendingTransaction() with AsyncStorage cleanup
• Implement checkPendingTransactions() on app start
• Integrate with existing RootStore pattern

Acceptance Criteria

• Store persists pending transactions across app restarts
• Successfully tracks transaction status changes
• Handles app closure and restart gracefully
• Integrates with existing MobX architecture

Related Documentation

• See docs/POOL_CREATION_ARCHITECTURE.md Phase 4, Scenario B
• See docs/POOL_CREATION_TASKS.md Task 9

Labels: comp: frontend, type: feature

Milestone: Sprint 3: Pool Creation

Part of Epic: #35