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/color_select.rs

@@ -125,7 +125,7 @@ pub fn color_selector<'a>(
         row![
             swatch(cur_bg),
             text(cur_name).size(11).color(TEXT),
-            text(if open { " ▲" } else { " ▾" }).size(9).color(TEXT),
+            crate::ui::icons::arrow_toggle(open, 9.0, TEXT),
         ]
         .spacing(5)
         .align_y(iced::Center),