Refactored files.ts

[Klakurka]

Related to #981

Description

Refactored the 150+ line collapseSmallPayments function in utils/files.ts to improve maintainability and readability. Extracted complex logic into a TransactionGroupManager class and utility functions (generateGroupKey, extractPaybuttonNames, etc.). The main function is now ~40 lines with clearer control flow. No breaking changes - public API and functionality remain identical.

Test plan (suggested; go through whatever you think is worth checking)

Automated Testing

• Run existing unit tests: npm test tests/unittests/utils/files.test.ts
• Verify TypeScript compilation: npx tsc --noEmit
• Run full test suite: npm test

Functional Testing

• CSV export with small payments (below threshold) - verify collapsing behavior
• CSV export with large payments (above threshold) - verify individual processing
• Mixed payment scenarios with both small and large transactions
• Multi-button paybutton name extraction
• Different timezone handling for date grouping
• Multiple currency support (USD, CAD, etc.)

Edge Cases

• Empty payment arrays
• Single transaction processing
• Price validation error handling
• Invalid paybutton ID references

Integration Testing

• Complete CSV download flow through API endpoints
• Database transaction retrieval and processing
• User permission filtering by paybutton ownership

[chedieck]

I think this excess of comments pollute the codebase and makes it hard to read, better to ask the AI to only comment when it thinks it to be really necessary.

[chedieck]

Seems good to me, better wait for @lissavxo review since she created the original implementation.

[Klakurka]

Lissa had approved already.