Web files: designs live in the browser (OPFS), files come in by picker or drop and go out as downloads

- solvecraft_io::vfs: the engine's file access (disk natively; in the browser an in-memory set of
  files the host loads and stores). File commands and write_atomic go through it.
- Web host (apps/solvecraft-web/src/files.rs): stored designs are read from OPFS at start
  (storage.persist() asked for) and written back when a command writes one; Save/Save As keep
  designs in the browser, exports (STEP, STL, 3MF, OBJ, ...) download as files.
- File > Open shows "Files in this browser": open, insert, rename, download, delete, plus open or
  insert from the computer (showOpenFilePicker where there is one, an <input type=file>
  otherwise). Files dropped on the page are inserted or opened.
- Unsaved work is autosaved to OPFS and offered again after a reload or crash.
- Start page recents come from the stored designs, with thumbnails. In the browser the samples
  are built when opened (a picture blocks the page there, and a kernel panic can't be caught).
- Pages without OPFS (http on a network address, old browsers) keep designs in local storage.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/io/src/safe_write.rs

@@ -87,6 +87,11 @@ fn write_steps(path: &Path, bytes: &[u8], backup: bool, stop: Option<Step>) -> s
 /// Write `bytes` to `path` atomically; with `backup`, the file being replaced is kept as
 /// `<path>.bak` first.
 pub fn write_atomic(path: &Path, bytes: &[u8], backup: bool) -> std::io::Result<()> {
+    // In the browser the host stores files (one write is already atomic there).
+    if cfg!(target_arch = "wasm32") {
+        let _ = backup;
+        return crate::vfs::write(&path.to_string_lossy(), bytes);
+    }
     write_steps(path, bytes, backup, None)
 }