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/plot.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="64" height="64">
+  <path d="M7 9 V3 H17 V9" stroke="#e0e0e0" stroke-width="1.7" fill="none" stroke-linejoin="round" stroke-linecap="round"/>
+  <path d="M7 17 H4 V10 a1 1 0 0 1 1-1 H19 a1 1 0 0 1 1 1 V17 H17" stroke="#e0e0e0" stroke-width="1.7" fill="none" stroke-linejoin="round" stroke-linecap="round"/>
+  <rect x="7" y="14" width="10" height="7" stroke="#4cc9f0" stroke-width="1.5" fill="none" stroke-linejoin="round" stroke-linecap="round"/>
+</svg>