test: cover interface form controls

[Yassinbrine]

/claim #71

Summary

• Add focused Vitest coverage for the interface Button and Select controls.
• Cover Button anchor rendering, variant/size/custom classes, loading state, disabled state, and square icon-only behavior.
• Cover Select placeholder/options/value rendering plus size and custom classes.

Verification

• pnpm exec vitest run plugins/interface/components/form-controls.test.tsx --config .tools/vitest.empty.mjs -> 3 tests passed
• pnpm exec prettier --check plugins/interface/components/form-controls.test.tsx -> passed
• git diff --check -> passed

Note: the temporary .tools/vitest.empty.mjs config was used only for local verification because this Windows OneDrive workspace hit Vite's TS config temp-file ENOENT before tests started. The added test file itself is standard Vitest/Hono JSX and should run under the repository config in CI.