feat(billing): add per-user search to billing breakdown

[karpikpl]

Fixes #434

Summary

• add a search field above the Billing per-user breakdown table
• bind Vuetify table search to the user column only
• lazy-load billing for the first matched users when searching
• add Vitest coverage for the search wiring and lazy-load path

Validation

• npm test
• npm run build
• npm run lint (fails before linting due dependency ESM export error: brace-expansion missing expand export)