web: enable STL/STEP export and OBJ import, drag-and-drop open

STLOUT/STEPOUT wrote via std::fs and IMPORTOBJ read via std::fs, all
of which fail on wasm32. The export pickers now mirror the PDF path
(pick_*_owned returns a name-only PathBuf; the browser download names
the file) and the write tail swaps fs::write for sys::download_bytes.
OBJ import reads the picked file's bytes directly, like the web open
picker.

Opening by drag-and-drop was dead on the web build: winit never emits
FileDropped there. A wasm-gated io::web_drop module installs DOM
dragover/drop listeners, filters .dwg/.dxf, reads the File bytes
async, and queues them; the existing 50 ms PollWebControl tick drains
the queue into the same open path the picker uses (progress overlay,
recovery, OPFS recent cache).

Also floors the model-space plot sheet: extents smaller than 10 mm
(or the 0x0 paper a fresh unsaved drawing carries) now fall back to
the standard sheet instead of producing a ~3 pt page.

(cherry picked from commit ad60be11ed)

src/io/mod.rs

@@ -31,6 +31,8 @@ pub mod visual_styles;
 pub(crate) mod web_recent;
 #[cfg(target_arch = "wasm32")]
 mod web_worker;
+#[cfg(target_arch = "wasm32")]
+pub(crate) mod web_drop;
 pub mod windows_media;
 pub mod xref;
 pub mod xref_model;