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/trash.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#000000" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round"><path d="M4 7 H20 M9 7 V5 a1 1 0 0 1 1-1 H14 a1 1 0 0 1 1 1 V7 M6 7 L7 20 a1 1 0 0 0 1 1 H16 a1 1 0 0 0 1-1 L18 7 M10 11 V17 M14 11 V17"/></svg>