feat(web): zoom controls for workflow diagram

[ikarys]

Summary

The workflow diagram in the editor scaled to container width and grew unboundedly with step count, making large workflows impossible to view whole. This adds zoom controls and fit-to-screen so the entire diagram is always navigable.

Changes

• Fit-to-screen on open and workflow switch — entire diagram visible inside the panel without scrollbars, regardless of step count.
• Zoom controls overlay (zoom in, zoom out, fit-to-screen) with bounded zoom (0.1×–4×). Hidden when there are no steps.
• Ctrl/Cmd + wheel zoom toward the cursor (point under cursor stays fixed); page/modal does not scroll while zooming.
• Scroll as pan — existing vertical scroll preserved; horizontal scroll enabled when zoomed in (ScrollArea `both`).
• Zoom works in both read-only and edit modes; transition create/reconnect/delete remain disabled in read-only.
• Existing interactions stay pixel-accurate at any zoom level: node/edge selection, drag-from-port, reconnect handles, delete-on-edge-key. Achieved by keeping the SVG `viewBox` fixed to logical coords and setting the rendered size via CSS (`width × zoom`), so `getSVGPoint` (which uses `getScreenCTM().inverse()`) stays correct.

Implementation

• New `useWorkflowZoom` hook reads the SVG viewBox via ref (no duplicate `computeLayout` call). Exposes `zoomIn`/`zoomOut`/`fitToScreen` and derived `canZoomIn`/`canZoomOut`/`isFitted` flags (dumb client).
• `FlowDiagram` uses `forwardRef` to expose the SVG element; fixed `viewBox` + explicit CSS size.
• New `MinusIcon` and `ZoomFitIcon` components.

AI-Enhanced Development

• AI Models: GLM 5.2 via OpenFox agent

Cache Impact

• No — no changes to system prompts, tool definitions, skills, or cached context.