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/page_setup.rs

@@ -329,12 +329,12 @@ pub fn view_window<'a>(
         hdivider(),
         section_label("Position"),
         button(
-            text(if center {
-                "✓ Center on page"
-            } else {
-                "  Center on page"
-            })
-            .size(11)
+            row![
+                crate::ui::icons::check_cell(center, Color::WHITE),
+                text("Center on page").size(11),
+            ]
+            .spacing(2)
+            .align_y(iced::Center)
         )
         .on_press(Message::PageSetupCenterToggle)
         .style(pill(center))