fix(formulus): sync screen shows correct pending status

[Mishael-2584]

Description

Fixes the sync screen showing "All synced" when there are unsynced observations (#517).

Problem: After creating an observation and opening the Sync tab, the app could still show "All synced" and 0 pending. The count sometimes updated only later.

Cause:

1. Pending count was only updated on initial mount, not when returning to the Sync screen.
2. New observations can have synced_at stored as 0 instead of null; the pending query only checked for null.

Changes:

• SyncScreen: Use useFocusEffect to run updatePendingUploads() and updatePendingObservations() whenever the Sync screen gains focus, so the status and counts stay correct after creating observations or switching tabs.
• WatermelonDBRepo.getPendingChanges(): Add Q.where('synced_at', 0) next to the existing Q.eq(null) so never-synced observations are included whether the DB stores synced_at as null or 0. Comment updated to describe what “pending” includes.

Type of Change

• Bug Fix
• New Feature / Enhancement
• Refactor / Code Cleanup
• Documentation Update
• Maintenance / Chore
• Other (please specify):

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Component(s) Affected

• formulus (React Native mobile app)
• formulus-formplayer (React web app)
• synkronus (Go backend server)
• synkronus-cli (Command-line utility)
• Documentation
• DevOps / CI/CD
• Other:

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Related Issue(s)

Closes/Fixes/Resolves: Closes #517

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Testing

• Unit tests added/updated
• Integration tests added/updated
• Manually tested
• Tested on multiple platforms (if applicable)
• Not applicable

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Breaking Changes

• This PR introduces breaking changes
• This PR does NOT introduce breaking changes

If breaking changes, please describe migration steps: N/A

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Documentation Updates

• Documentation has been updated
• Documentation update is not required

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Checklist

• Code follows project style guidelines
• All existing tests pass
• New tests added for new functionality
• PR title follows Conventional Commits format

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