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/manage/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()),
     }
 }

src/modules/manage/options.rs

@@ -4,7 +4,7 @@ pub fn tool() -> ToolDef {
     ToolDef {
         id: "OPTIONS",
         label: "Options",
-        icon: IconKind::Glyph("⚙"),
+        icon: IconKind::Svg(include_bytes!("../../../assets/icons/options_tool.svg")),
         event: ModuleEvent::Command("OPTIONS".to_string()),
     }
 }