fix: diagram rendering improvements

- Recursive compound rendering: CompoundNode checks if children are
  themselves compound types (WHEN inside CHOICE) and renders them
  recursively. Added EIP_WHEN, EIP_OTHERWISE, DO_CATCH, DO_FINALLY
  to frontend COMPOUND_TYPES.
- Edge z-ordering: edges are distributed to their containing compound
  and rendered after the background rect, so they're not hidden behind
  compound containers.
- Error section sizing: normalize error handler node coordinates to
  start at (0,0), compute red tint background height from actual
  content with symmetric padding for vertical centering.
- Toolbar as HTML overlay: moved from SVG foreignObject to absolute-
  positioned HTML div so it stays fixed size at any zoom level. Uses
  design system tokens for consistent styling.
- Zoom: replaced viewBox approach with CSS transform on content group.
  Default zoom is 100% anchored top-left. Fit-to-view still available
  via button.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/ProcessDiagram/ProcessDiagram.module.css

@@ -77,3 +77,39 @@
   text-align: center;
   font-variant-numeric: tabular-nums;
 }
+
+.nodeToolbar {
+  position: absolute;
+  display: flex;
+  align-items: center;
+  gap: 2px;
+  padding: 3px 4px;
+  background: var(--bg-surface, #FFFFFF);
+  border: 1px solid var(--border, #E4DFD8);
+  border-radius: var(--radius-sm, 5px);
+  box-shadow: var(--shadow-lg, 0 4px 16px rgba(44, 37, 32, 0.10));
+  transform: translate(-50%, -100%);
+  margin-top: -6px;
+  z-index: 10;
+  pointer-events: auto;
+}
+
+.nodeToolbarBtn {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 26px;
+  height: 26px;
+  border: none;
+  background: transparent;
+  color: var(--text-secondary, #5C5347);
+  font-size: 12px;
+  cursor: pointer;
+  border-radius: var(--radius-sm, 5px);
+  padding: 0;
+}
+
+.nodeToolbarBtn:hover {
+  background: var(--bg-hover, #F5F0EA);
+  color: var(--text-primary, #1A1612);
+}