fix(app): keep server details editable

[opencode-agent]

Summary

• keep the server address controlled for validation and health preview behavior
• make the optional server name, username, and password fields uncontrolled so Kobalte cannot roll back typed characters on Windows/Electron
• scope the workaround to the affected Add/Edit server form instead of changing the shared TextField component

Verification

• bun typecheck in packages/app
• git diff --check

Fixes #38193

Requested by: @Hona (Hona via Slack)