Add search filters to management tables

[artrmkr]

Fixes #3

Summary

• Restore the shared navbar search input and wire it to controlled page state.
• Add client-side filtering for Items by id, owner id, title, and description.
• Add client-side filtering for Admin users by id, name, email, role, and status.
• Show a clear empty-results row instead of an empty table when a search has no matches.

Verification

• npm run build
• biome check --no-errors-on-unmatched --files-ignore-unknown=true src/components/Common/Navbar.tsx src/routes/_layout/admin.tsx src/routes/_layout/items.tsx src/utils.ts
• git diff --check

Note: npm run build still reports the existing Vite chunk-size warning, but the build exits successfully.