feat: laid out "add primitive" buttons

assets/icons/plus.svg

@@ -0,0 +1,57 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!-- Created with Inkscape (http://www.inkscape.org/) -->
+
+<svg
+   width="16.933332mm"
+   height="16.933332mm"
+   viewBox="0 0 16.933332 16.933332"
+   version="1.1"
+   id="svg1"
+   inkscape:version="1.4.2 (ebf0e940d0, 2025-05-08)"
+   sodipodi:docname="plus.svg"
+   inkscape:export-filename="../../project/assets/icons/plus.svg"
+   inkscape:export-xdpi="96"
+   inkscape:export-ydpi="96"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg">
+  <sodipodi:namedview
+     id="namedview1"
+     pagecolor="#ffffff"
+     bordercolor="#999999"
+     borderopacity="1"
+     inkscape:showpageshadow="2"
+     inkscape:pageopacity="0"
+     inkscape:pagecheckerboard="0"
+     inkscape:deskcolor="#d1d1d1"
+     inkscape:document-units="mm"
+     inkscape:zoom="11.046421"
+     inkscape:cx="33.766593"
+     inkscape:cy="34.852918"
+     inkscape:current-layer="layer1" />
+  <defs
+     id="defs1" />
+  <g
+     inkscape:label="Layer 1"
+     inkscape:groupmode="layer"
+     id="layer1">
+    <rect
+       style="fill:#969696;fill-opacity:1;stroke:none;stroke-width:5;stroke-linecap:round;paint-order:fill markers stroke"
+       id="rect1"
+       width="3.4427652"
+       height="12.431063"
+       x="6.7452841"
+       y="2.2511349"
+       ry="0.49476871" />
+    <rect
+       style="fill:#969696;fill-opacity:1;stroke:none;stroke-width:5;stroke-linecap:round;paint-order:fill markers stroke"
+       id="rect2"
+       width="3.4427652"
+       height="12.431063"
+       x="6.7452841"
+       y="-14.682198"
+       ry="0.49476871"
+       transform="rotate(90)" />
+  </g>
+</svg>