fix: accrue interest with bps denominator

[silentgeckoaudit3801]

Closes #1307

Summary:

• Use the standard 10,000 basis-point denominator for interest accrual instead of 100,000.
• Add a regression test for a 10,000 principal loan at 1,200 bps over one full default term, expecting 1,200 accrued interest.

Validation:

• Static GitHub API/source inspection only; local Soroban/Rust toolchain commands were not run to avoid installing or executing project dependencies in this environment.

[ogazboiz]

thanks for the PR. this was a real bug when you branched, but the same fix already landed on main: loan_manager lib.rs:359 already accrues with the 10_000 bps denominator. since the change is now redundant, closing this one to keep the queue clean. please branch from current main before your next fix PR, and feel free to reopen if I misread anything. if you want to keep contributing, join us on Telegram: https://t.me/+DOylgFv1jyJlNzM0