Currency handling improvements

[krokosik]

Closes #412 #411

Description

Implemented a proper currency formatter based on Intl browser API. We now properly render decimals and fixed numerous issues with the current implementation, superceding PRs #413 #419

The case where the split is not properly defined (for example in exact split) is now handled more gracefully in the UI with nonsensical share texts hidden. Additionally a critical performance issue was fixed with large sums and leftover pennies calculation, which froze the webpage on huge leftover sums.

The simplify method has been patched to properly fill in metadata of generated simplified balances that did not exist prior, causing a crash on balanceList.

What is more, the recalculate method also now better deals with malformed balance data, where not all group balance relations are created, since newly joined users may not have groupBalances with everyone (another case for #247)

Demo

screenrecording-2025-10-19_09-53-08.mp4

Checklist

• I have read CONTRIBUTING.md in its entirety
• I have performed a self-review of my own code
• I have added unit tests to cover my changes
• The last commit successfully passed pre-commit checks
• Any AI code was thoroughly reviewed by me