docs: explain browser saves and local downloads

docs/book/src/guide/files.md

@@ -18,7 +18,9 @@ replaced version is kept as `<file>.bak`. Unsaved changes are autosaved to a rec
 and the next launch offers to recover them. Older design files are upgraded on open, and files
 from a newer version are refused with a message.
 
-Current designs use format 10 for recorded body transfers, explicit 3D curve coordinate
+Current designs use format 12, including persistent analytic offset boundary roles and
+association profile references, durable sketch face anchors, periodic fit splines and versioned
+revolve conventions. It also retains recorded body transfers, explicit 3D curve coordinate
 spaces and certified face bindings, alongside semantic configuration styles, associative
 free-form offsets and cylindrical emboss selections. Older ownership maps, geometry and
 expressions retain their saved meaning. Legacy 3D curves with retained associations receive
@@ -26,3 +28,21 @@ their producer's coordinate-space marker without changing any saved points; deta
 retain world coordinates. A lost cross-component association refreshed by an older build may
 need an explicit relink or refresh when its source returns. Loading never guesses a numerical
 correction. Association records, coordinate arrays and placements are bounded and checked.
+
+## Saving in the web app
+
+In the web app, **Save** and **Save As** keep designs in this browser's private storage;
+they do not open a dialog for choosing a folder on your computer. Clearing the site's data
+can remove those designs. Download a copy to keep it outside the browser or move it to
+another device.
+
+1. Open the File menu and choose **Open** to show **Files in this browser**.
+2. Choose **Download open design** for the current design, including its unsaved changes,
+   or **Download** beside a stored file for its last saved contents.
+3. Find the `.solvecraft` file in your browser's downloads. Whether a folder chooser appears
+   depends on your browser's download settings.
+
+Use **Open from computer…** in the same panel to reopen a downloaded design. The web app
+works on a browser-stored copy; later saves do not update the original downloaded file.
+Use **Rename** beside a stored file to change its name. Exports such as STEP and STL are
+also downloaded.