Improve diagram editor connection compatibility hints

[ArizmendiWan]

This PR is an initial implementation for #172. It improves drag-time connection feedback and adds a compatibility-aware next-operation flow in the diagram editor.

Current changes include:

• clearer compatible/incompatible handle feedback while dragging
• compatibility detection for drags starting from either source or target handles
• dropping on empty space to open a filtered "Compatible next operations" popup

demo.mov

This PR also includes a small incidental frontend sync fix for a stale generated type/schema reference that was needed to keep the frontend building cleanly.

GenAI Use

• [√ ] I used a GenAI tool in this PR.
• [ ] I did not use GenAI

Generated-by: Codex Version 26.309.31024 (962) and Claude Code v2.1.76.

[mxgrey]

Thanks for this contribution! The UI hints shown in the video look very impressive and helpful!

Just one high level thought before conducting a full code review: a Clear Diagram button in the main menu bar is extremely destructive. There's a risk a user could accidentally click on that while trying to do something else. I would recommend removing that at least until we have an undo feature for the editor.

[aaronchongth]

Thanks for the contribution. I've just left some high level review comments for now. Will get back to you soon with more a more in-depth review

[ArizmendiWan]

Thanks for your feedbacks! I have addressed the requested review changes:

• removed the Clear Diagram action
• moved the connection helper from top-right to top-left
• removed the handbook roadmap addition
• updated the PR description to mention the incidental stale generated type/schema sync fix

demo_v01.mov

Please let me know if there’s anything else you’d like me to adjust.

[aaronchongth]

Thanks for the changes so far, I left a few comments.

Regarding the UI, the buttons should appear below the filter input, instead of on the right.

When there are no valid operations, the text lines up below the filter properly, the buttons should do that too in general

[ArizmendiWan]

Thanks for the review! I have addressed the remaining review comments:

• Replaced the inline section node type checks with the existing helpers for consistency.
• Updated the empty-state messaging to distinguish filter-empty from compatibility-empty.
• Also added support for reverse-flow compatibility so the compatibility UI now works when starting a drag from either direction.

demo_v02.mov

[ArizmendiWan]

I also updated the compatibility hint functionality. Right now it also checks if a handle is already full. If one handle is full, no compatibility will be shown and the hint panel will show corresponding messages.

demo_v03.mov

[aaronchongth]

Thanks for the added features and validations! The validations and connection restrictions make sense to me, and I didn't manage to trigger any edge cases so far, but I'll leave the final verdict to @mxgrey to see if anything comes to mind that may be an edge case.