fix(web): render UI icons as SVG instead of font glyphs

UI chrome (dropdown carets, undo/redo, checkmarks, close/menu/osnap,
layer/file-menu icons) and ribbon tool icons were drawn as Unicode
glyphs. Desktop fills missing glyphs via system-font fallback, but the
web build bundles only Fira Sans with no fallback, so they rendered as
empty tofu boxes (issue #116).

Add a shared src/ui/icons.rs (SVG-backed, tintable) plus bundled assets,
and switch every glyph site and IconKind::Glyph tool def to SVG.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/icons/ui/redo.svg

@@ -0,0 +1,43 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   viewBox="0 0 24 24"
+   fill="none"
+   stroke="#000000"
+   stroke-width="2.2"
+   stroke-linecap="round"
+   stroke-linejoin="round"
+   version="1.1"
+   id="svg2"
+   sodipodi:docname="redo.svg"
+   inkscape:version="1.4.4 (dcaf3e7d9e, 2026-05-05)"
+   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">
+  <defs
+     id="defs2" />
+  <sodipodi:namedview
+     id="namedview2"
+     pagecolor="#505050"
+     bordercolor="#eeeeee"
+     borderopacity="1"
+     inkscape:showpageshadow="0"
+     inkscape:pageopacity="0"
+     inkscape:pagecheckerboard="0"
+     inkscape:deskcolor="#505050"
+     inkscape:zoom="17.948527"
+     inkscape:cx="22.007378"
+     inkscape:cy="9.4436721"
+     inkscape:window-width="1920"
+     inkscape:window-height="939"
+     inkscape:window-x="0"
+     inkscape:window-y="0"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg2" />
+  <path
+     d="m 16,3 5,5 -5,5"
+     id="path1" />
+  <path
+     d="M 21,8 H 10 a 6,6 0 0 0 0,12 h 2"
+     id="path2" />
+</svg>