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/mod.rs

@@ -14,13 +14,13 @@ pub fn paper_space_tools() -> Vec<ToolDef> {
         ToolDef {
             id: "PAGESETUP",
             label: "Page Setup",
-            icon: IconKind::Glyph("📋"),
+            icon: IconKind::Svg(include_bytes!("../../../assets/icons/pagesetup.svg")),
             event: ModuleEvent::Command("PAGESETUP".to_string()),
         },
         ToolDef {
             id: "PLOT",
             label: "Export PDF",
-            icon: IconKind::Glyph("🖨"),
+            icon: IconKind::Svg(include_bytes!("../../../assets/icons/plot.svg")),
             event: ModuleEvent::Command("PLOT".to_string()),
         },
     ]
@@ -48,14 +48,14 @@ impl CadModule for LayoutModule {
                     ToolDef {
                         id: "PAGESETUP",
                         label: "Page Setup",
-                        icon: IconKind::Glyph("📋"),
+                        icon: IconKind::Svg(include_bytes!("../../../assets/icons/pagesetup.svg")),
                         event: ModuleEvent::Command("PAGESETUP".to_string()),
                     }
                     .into(),
                     ToolDef {
                         id: "PLOT",
                         label: "Export PDF",
-                        icon: IconKind::Glyph("🖨"),
+                        icon: IconKind::Svg(include_bytes!("../../../assets/icons/plot.svg")),
                         event: ModuleEvent::Command("PLOT".to_string()),
                     }
                     .into(),

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()),
     }
 }