fix: send full repayment amount instead of one-tenth in buildUnsigned…

[extolkom]

Summary

Closes #1368

Fixes a critical bug where buildUnsignedRepaymentXdr in
frontend/src/app/utils/soroban.ts divided the repayment amount by
ten before building the XDR, allowing a borrower to have their loan
marked as fully repaid while only transferring 10% of the actual owed
amount.

Root cause

[exact line/description of the bug — e.g. "the function applied an incorrect stroop conversion, dividing by 10 where it should have used the correct decimal-precision constant"], found at
frontend/src/app/utils/soroban.ts:[line number].

Fix

[one-line description of the actual fix — e.g. "corrected the decimal conversion to match the pattern already used in [sibling function name], which converts amounts correctly"]

Scope

• Only buildUnsignedRepaymentXdr and its direct conversion logic
  were touched — no other functions, no refactors
• No changes to the function signature or parameters
• Per the issue's own contributor note, this PR fixes only this one
  issue/function; no other bugs were introduced or fixed here

Tests added/updated

[test file path] — asserts that for a known input repayment amount,
the resulting XDR/on-chain value equals the full input amount, not
one-tenth of it.

Verification

• npm run build — [pass/fail]
• npm run lint — [pass/fail]
• npm test — [pass/fail summary]
• Manually verified: [state what you personally checked — e.g. "for a 100 USDC repayment input, the built XDR now encodes 100 USDC, not 10 USDC, confirmed by decoding the resulting XDR"]
• Branch rebased on latest origin/main

Notes for reviewers

This is a financial-precision fix, so I'd ask for a careful review of
the decimal-conversion logic specifically — [flag anything relevant, e.g. "confirm this is correct across all supported asset decimal precisions used in this contract, not just the one tested"]. Given
the severity (borrowers could clear loans while paying 10% of what's
owed), I'd treat this as a priority merge once verified.