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>

src/ui/mod.rs

@@ -8,6 +8,7 @@ pub mod color_select;
 pub mod command_line;
 pub mod cycle_popup;
 pub mod dimstyle;
+pub mod icons;
 pub mod isolate_popup;
 pub mod layers;
 pub mod layout_manager;