[Testing] Unit-test the data and contract hooks and stores, wire a coverage gate

[grantfox-oss]

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Why this matters

There are only a handful of unit tests for roughly 130 source files, and the most failure-prone logic (transaction build and submit hooks, the wallet and user stores) has none. Without coverage these regress silently. This is the safety net the rest of the frontend work needs.

Acceptance criteria

• Add unit tests for useApi, useContractMutation, useConfirmedMutation, useTransactionPreview, and useRepaymentOperation
• Add store tests for useWalletStore and useUserStore covering connect, switch, disconnect, and error paths
• Mock the Stellar SDK and fetch so tests are deterministic and offline
• Enable Jest coverage with a sensible initial threshold (ratchet, do not lower) and run it in CI

Files to touch

• src/app/hooks/*.ts
• src/app/stores/*.ts
• jest.config.js (coverage config and thresholds)

Out of scope

• E2E flows (tracked separately)
• Visual regression

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #6)
2. Your PR will be reviewed by the LabsCrypt maintainers

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