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 <[email protected]>

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};
-    let secs = SystemTime::now()
-        .duration_since(UNIX_EPOCH)
-        .map(|d| d.as_secs())
-        .unwrap_or(0);
+    // `SystemTime::now` panics in the browser; ask the page's clock there. (#761)
+    #[cfg(target_arch = "wasm32")]
+    let secs = (js_sys::Date::now() / 1000.0) as u64;
+    #[cfg(not(target_arch = "wasm32"))]
+    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;