test: add interface plugin coverage

[popsiclelmlm]

Supersedes #185 with the same interface coverage change from a clearer source branch.

/claim #71

Summary

• Add focused Vitest coverage for InterfacePlugin route registration, route matching, dataSource capture, and template page rendering.
• Add interface utility coverage for class merging and Vite manifest asset tag generation.
• Add server-render coverage for Avatar, Button, Card, Input, Label, Loader, Select, and Toggle components.

Tests

• ./node_modules/.bin/vitest --run plugins/interface/index.test.tsx plugins/interface/utils/index.test.tsx plugins/interface/components/avatar/index.test.tsx plugins/interface/components/button/Button.test.tsx plugins/interface/components/card/index.test.tsx plugins/interface/components/input/Input.test.tsx plugins/interface/components/label/Label.test.tsx plugins/interface/components/loader/Loader.test.tsx plugins/interface/components/select/index.test.tsx plugins/interface/components/toggle/index.test.tsx
• ./node_modules/.bin/vitest --run (new tests pass; existing upstream RLS assertions in src/rls/index.test.ts still fail independently)