fix: reject unsubmittable participant shares in the percentage UI

[blesswill88]

Overview

The percentage UI validated only the total. Because shares are basis-point
integers, a total of exactly 10000 said nothing about whether the individual
shares were submittable:

const totalBasisPoints = participants.reduce((sum, p) => sum + p.basisPoints, 0);
const isValid = totalBasisPoints === BASIS_POINTS_TOTAL;

[10500, -500] sums to 10000, so the form showed a green "Shares total 100%."
and let the user submit a split where one participant is owed a negative amount
and another is above 100%. The server then rejected it — the UI's job is to
catch that first. Fractional and non-numeric shares slipped through the same
way.

Related Issue

Closes #1321

Changes

• ParticipantPercentageValidation.tsx
  • Each share is now checked individually: it must be a finite, whole,
    non-negative number of basis points not exceeding 10000. Violations are
    reported as invalidShares with an index, label and machine-readable
    problem (not_a_number, not_whole_basis_points, negative,
    above_total).
  • An impossible share contributes nothing to the total instead of being
    added; otherwise two broken shares could mask each other, which is exactly
    the bug above.
  • isValid now requires both no invalid shares and a total of 10000. The
    message names the offending participant(s) and counts the remainder, so the
    user is told which row to fix rather than only that the sum is wrong.
  • The panel lists the broken shares and keeps the existing
    data-testid/data-valid hooks and role="status" semantics.
  • onValidityChange moved out of useMemo into useEffect. Calling the
    parent's setter during the render phase is a side effect in render — React
    warns about it and it could loop when the parent's state fed back in — and
    it fired on every keystroke even when validity had not changed.
• ParticipantPercentageValidation.test.ts — adds the per-share cases: a
  legal total built from illegal shares, fractional and non-numeric shares,
  message wording, the zero-share exclusion, and a missing list. The four
  original tests are unchanged apart from asserting invalidShares.

Verification Results

$ npx vitest run src/components/splits/ParticipantPercentageValidation.test.ts
 ✓ src/components/splits/ParticipantPercentageValidation.test.ts (13 tests)
 Test Files  1 passed (1)
      Tests  13 passed (13)

Not run in this environment: the React render behaviour of the `useEffect`
change, and the rest of the frontend suite (the change was applied through the
GitHub Contents API with no local clone, so no browser/RTL run was possible).

Acceptance Criteria	Status
Show percentage totals	✅ unchanged total readout, now excluding unsubmittable shares
Highlight invalid totals	✅ red styling preserved and extended to individual shares
Prevent invalid submission	✅ shouldBlockSubmit/isValid reject impossible shares even when the total is 100%
Server validation stays authoritative	✅ client is a pre-check only; the server remains the enforcement point

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[SheyeJDev]

LGTM