fix(proxy): correctly clear plugin config when disabled

[YinMo19]

Summary

• fix tunnel proxy forms so plugin data is only submitted when plugin switch is enabled
• clear in-memory pluginConfig when disabling "Use Plugin"
• pass current pluginConfig to plugin form to avoid restoring stale default plugin data

Scope

• TCP / STCP / UDP / HTTP proxy forms

Validation

• pnpm lint
• pnpm exec tsc --noEmit