Add search filters to management tables

[randyxian08]

Implements the global search entry point requested in #3 by wiring the existing navbar search UI into the main management tables.

Changes:

• Restores the navbar search input with the existing search icon
• Filters Items by id, title, and description
• Filters Admin users by name, email, role, and status
• Shows an empty-state row when no records match

Validation:

• npm run build
• npx biome check --no-errors-on-unmatched src/components/Common/Navbar.tsx src/routes/_layout/items.tsx src/routes/_layout/admin.tsx

Closes #3