fix(autosave): autosave unsaved drawings + clean up .sv$ on file-close (#205)

Two gaps in the autosave recovery files:

1. A brand-new, never-saved drawing was never autosaved — on_autosave only
   wrote tabs that already had a path. A shared autosave_target() now writes
   saved drawings to <file>.sv$ and unsaved ones to the system temp dir
   (OpenCADStudio_<name>.sv$).

2. Closing a file (tab) left its .sv$ behind, which also defeated the
   program-close cleanup (it only scans still-open tabs). on_tab_close and the
   unsaved-discard path now delete the closing tab's .sv$, and Save/Save-As
   drop the temp copy a drawing used while it was still unsaved.

Program-close cleanup (exit_app -> cleanup_autosaves) already worked.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/command.rs

@@ -31,6 +31,9 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
                     self.pending_close = Some(crate::app::PendingClose::Tab(idx));
                     return self.open_unsaved_dialog_window();
                 }
+                // This tab is closing for good — drop its autosave recovery copy.
+                #[cfg(not(target_arch = "wasm32"))]
+                let _ = std::fs::remove_file(self.autosave_target(idx));
                 // Only-tab case: when the lone non-start tab closes, fall
                 // back to the Start tab if it exists; otherwise spawn a
                 // fresh blank drawing (legacy behaviour).