fix: save percentage calculations to history

[Guflly]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

• Percentage operations now save their formula and result through the same history path as normal calculations.
• Reused the history insertion logic to keep both paths consistent.

Tests performed

• ./gradlew detekt
• Compiled the affected production classes in an isolated JVM harness and verified 85 × 25 % creates one history entry with formula 85×25% and result 21.25.

Closes the following issue(s)

• Closes Percentual (%) operations not saved in history #304

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.