fix: reveal off-screen auth values

[thangldw]

Summary

• make authentication field tables horizontally scrollable when their value column exceeds the available width
• show a clear “Scroll for values →” cue only while content remains hidden to the right
• keep read-only/inherited auth tables scrollable and expose the region with an accessible label and focus target
• reuse the same behavior for OAuth 2.0 and the other auth types

Fixes VoidenHQ/voiden#497

Verification

• npm run build
• targeted TypeScript check for src/nodes/AuthNode.tsx
• host integration bundle: yarn node scripts/build-plugins.mjs voiden-advanced-auth --dev
• Prettier check and git diff --check
• boundary checks for overflowing, fully visible, and end-of-scroll states

The host repository’s full TypeScript check currently reports pre-existing errors outside this plugin; no reported error points to this change.