Give the centerline tool a toolbar icon, and paint both line-type icons in construction orange (#427)

Centerline gets a place in SKETCH > CREATE with its own long-short icon, and both line-type icons now draw in the construction orange.

crates/ui-egui/tests/scenarios/sketch_centerline_toolbar_icon.json

@@ -0,0 +1,20 @@
+[
+  {
+    "note": "Centerline needs the same CREATE panel placement and icon as its neighbours (sketch.ellipse, sketch.conic): without them it never appeared in the CREATE panel's drop-down at all, pinned or not. Both it and the construction toggle draw their icon in the app's fixed construction orange, not the tab or panel accent colour a caller happens to pass to icons::paint."
+  },
+  {"start": "sketch.create"},
+  {"click": {"plane": "XY"}},
+  {"expect": {"dialog": null}},
+  {"click": {"handle": "panel:SKETCH:CREATE"}},
+  {"expect": {"shown_handles": ["toolbar:sketch.construction", "toolbar:sketch.centerline"]}},
+  {"click": [1000, 500]},
+  {"start": "sketch.rectangle.two_point"},
+  {"click": {"sketch": [0, 0]}},
+  {"click": {"sketch": [30, 20]}},
+  {"key": "Escape"},
+  {"expect": {"query": {"command": "sketch.inspect", "path": ["profiles", 0, "area_mm2"], "value": 600}}},
+  {"start": "sketch.centerline"},
+  {"click": {"sketch": [15, 0]}},
+  {"key": "Escape"},
+  {"expect": {"query": {"command": "sketch.inspect", "path": ["profiles"], "value": []}}}
+]