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/units_popup.rs

@@ -73,10 +73,7 @@ pub fn units_popup_overlay(current: i16) -> Element<'static, Message> {
 }
 
 fn unit_row(label: &'static str, active: bool, msg: Message) -> Element<'static, Message> {
-    let check = text(if active { "✓" } else { "  " })
-        .size(11)
-        .color(if active { CHECK_COLOR } else { Color::TRANSPARENT })
-        .width(Length::Fixed(14.0));
+    let check = crate::ui::icons::check_cell(active, CHECK_COLOR);
 
     let lbl = text(label)
         .size(11)