feat(frontend): add employee performance bonus modal

[tokenjunkielabs]

Closes #187

Adds the employee-facing one-time performance bonus flow on top of PayD's existing payroll-bonus backend.

• clicking an employee opens an accessible performance-bonus modal;
• resolves that employee's organization and selects the next eligible draft/pending payroll run;
• shows the target batch, status, period start, and asset before committing the bonus;
• validates a positive amount and records an optional reason/description;
• submits to the existing idempotent /api/v1/payroll-bonus/items/bonus endpoint, so the bonus is stored as a bonus payroll item and included in the run totals;
• surfaces missing-run and request failures without silently attaching the bonus to a completed run.

The GrantFox claimant comment for tokenjunkielabs was published on #187 before implementation.

[tokenjunkielabs]

Maintainer review requested on this claimant-owned bounty carrier for #187. PR #615 is open, non-draft, and GitHub currently reports it mergeable at head d054595b7e320b19b0a9af4af5dbe399177eb0cb; there are no review comments on the carrier. Please review and merge if acceptable, or leave concrete change requests on this PR so the claimant branch can be updated directly.