feat: add reusable VaultFilterBar and filterVaults utility

[Tboy123-emm]

Summary

Closes #275

Introduces a reusable controlled VaultFilterBar component and a pure filterVaults utility to eliminate per-page re-implementation of status/search filtering.

Changes

• src/utils/filterVaults.ts: Pure filterVaults<T extends FilterableVault>(vaults, { status, query }) utility. Filters by VaultStatus | 'all' and case-insensitive name query (trims whitespace). Fully generic so it works for both Vaults and Dashboard vault arrays.
• src/components/VaultFilterBar.tsx: Controlled component accepting value: VaultFilters and onChange. Status options derived from VAULT_STATUS_ORDER (single source of truth). Token-styled using CSS variables. Keyboard accessible <select> and <input type="search">. Search landmark with role="search" and aria-label.
• src/pages/Vaults.tsx: Wired VaultFilterBar above the vault list in the data state; filter state managed with useState<VaultFilters>.
• src/utils/__tests__/filterVaults.test.ts: 12 test cases covering all/status/query filtering, case-insensitivity, whitespace trimming, empty list, generic types, and all five statuses.
• src/components/__tests__/VaultFilterBar.test.tsx: 8 test cases covering render, all options, controlled value reflection, onChange for select and input, search landmark, and keyboard accessibility.
• design-system/documentation/getting-started.md: Added VaultFilterBar section with usage example, props table, and accessibility notes.

Test Results

✓ src/utils/__tests__/filterVaults.test.ts    (12 tests)
✓ src/components/__tests__/VaultFilterBar.test.tsx  (8 tests)

Test Files  2 passed (2)
      Tests  20 passed (20)

The pre-existing failure in Vaults.test.tsx > shows data state when vaults exist was already present before this PR (confirmed by running the test on the base commit without these changes).

[drips-wave]

@Tboy123-emm Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[GBOYEE]

@GBOYEE has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Implement following existing patterns and conventions. Add tests, ensure CI passes, and document any new API surface.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @GBOYEE to this issue.