fix(web): STEP export no longer panics; STEP/STL exports download in the browser

The STEP header timestamp used SystemTime, which panics on wasm, and the
exports wrote through std::fs, which the browser does not have.

Fixes #761

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/update/file.rs

@@ -19,6 +19,21 @@ use codec::{EntityType as AcadEntityType, Handle};
 use iced::time::Instant;
 use iced::{mouse, Point, Task};
 
+/// Write an exported file: to disk on the desktop, as a browser download on
+/// the web, where there is no filesystem behind the dialog's path. (#761)
+fn write_export(path: &std::path::Path, bytes: &[u8]) -> Result<(), String> {
+    #[cfg(target_arch = "wasm32")]
+    {
+        let name = path.file_name().map_or_else(|| "export".into(), |n| n.to_string_lossy());
+        crate::sys::download_bytes(&name, bytes);
+        Ok(())
+    }
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        std::fs::write(path, bytes).map_err(|e| e.to_string())
+    }
+}
+
 pub(super) fn background_task<T, F, M>(work: F, map: M) -> Task<Message>
 where
     T: Send + 'static,
@@ -2006,7 +2021,7 @@ impl OpenCADStudio {
                 let mesh_refs: Vec<_> = meshes.iter().collect();
                 let bytes = crate::io::stl::build_stl(&mesh_refs)
                     .ok_or_else(|| "no mesh data to export".to_string())?;
-                std::fs::write(&worker_path, bytes).map_err(|e| e.to_string())
+                write_export(&worker_path, &bytes)
             },
             move |result| Message::StlExportFinished(path, result),
         )
@@ -2027,7 +2042,7 @@ impl OpenCADStudio {
                 let mesh_refs: Vec<_> = meshes.iter().collect();
                 let text = crate::io::step::build_step(&mesh_refs)
                     .ok_or_else(|| "no mesh data to export".to_string())?;
-                std::fs::write(&worker_path, text.as_bytes()).map_err(|e| e.to_string())
+                write_export(&worker_path, text.as_bytes())
             },
             move |result| Message::StepExportFinished(path, result),
         )

src/io/step.rs

@@ -255,11 +255,17 @@ pub fn build_step(meshes: &[&MeshModel]) -> Option<String> {
 /// Returns an ISO 8601-like timestamp string for the STEP file header.
 fn chrono_timestamp() -> String {
     // Use seconds since Unix epoch for a simple timestamp without chrono dep.
-    use std::time::{SystemTime, UNIX_EPOCH};
+    // `SystemTime::now` panics in the browser; ask the page's clock there. (#761)
-    let secs = SystemTime::now()
+    #[cfg(target_arch = "wasm32")]
-        .duration_since(UNIX_EPOCH)
+    let secs = (js_sys::Date::now() / 1000.0) as u64;
-        .map(|d| d.as_secs())
+    #[cfg(not(target_arch = "wasm32"))]
-        .unwrap_or(0);
+    let secs = {
+        use std::time::{SystemTime, UNIX_EPOCH};
+        SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .map(|d| d.as_secs())
+            .unwrap_or(0)
+    };
     // Format: YYYY-MM-DDTHH:MM:SS (approximate UTC from epoch seconds).
     let s = secs;
     let mins = s / 60;