fix(groups): require integer RPM overrides before adding

[ouchihao]

Clearing the new RPM input produces an empty string, which passes the existing null/negative checks. Fractions also pass because the Add button is outside a form. Both can enter the local list and fail the integer-valued save API. Require an integer in the existing button and handler checks, preserving zero as unlimited.

Validation:

• Covers empty, fractional and negative input, and successful saves with zero and a positive integer. 2 regression cases fail before the fix; all 5 focused cases pass after it.
• Focused tests plus the 17 Makefile critical frontend suites: 232 tests passed.
• vue-tsc --noEmit, full frontend ESLint and git diff --check passed.

Validation uses component tests with mocked APIs; no live server testing.