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/modules/layout/mview.rs

@@ -19,7 +19,7 @@ pub fn tool() -> ToolDef {
     ToolDef {
         id: "MVIEW",
         label: "Viewport",
-        icon: IconKind::Glyph("⬜"),
+        icon: IconKind::Svg(include_bytes!("../../../assets/icons/viewport.svg")),
         event: ModuleEvent::Command("MVIEW".to_string()),
     }
 }