web: autosave snapshots with crash-recovery restore, persistent storage

The web build had no autosave: on_autosave was a no-op and the
subscription was native-only. Both gates are gone; every savetime tick
now serializes each dirty tab (same pre-serialize sync as the web save
path, no thumbnail, serialized serially off the critical paint path)
into a dedicated opencadstudio-autosave OPFS directory, keyed by
drawing name, without touching the dirty flag, downloads, or recents.

On boot the web app scans that directory and offers any snapshot
newer than the last saved copy in a localized Recover Drawing modal
(Restore opens it as "name (recovered).dwg" through the standard
open pipeline; Discard removes it; Escape discards). Saving or
closing a drawing clears its snapshot; the boot scan also garbage-
collects stale entries. run_web now requests navigator.storage.persist
so settings and OPFS data survive browser eviction.

(cherry picked from commit 101d167ebc)

src/app/view/mod.rs

@@ -2460,9 +2460,9 @@ impl OpenCADStudio {
             iced::time::every(std::time::Duration::from_millis(300)).map(|_| Message::PollWebFonts);
         #[cfg(not(target_arch = "wasm32"))]
         let web_fonts = Subscription::none();
-        // Periodic autosave to a `.sv$` recovery file (native only). SAVETIME is
-        // the interval in minutes; 0 disables it.
-        #[cfg(not(target_arch = "wasm32"))]
+        // Periodic autosave (SAVETIME): a `.sv$` recovery copy beside the file
+        // on native, an OPFS snapshot on web. SAVETIME is the interval in
+        // minutes; 0 disables it.
         let autosave = if self.savetime_min > 0 {
             iced::time::every(std::time::Duration::from_secs(
                 self.savetime_min as u64 * 60,
@@ -2471,8 +2471,6 @@ impl OpenCADStudio {
         } else {
             Subscription::none()
         };
-        #[cfg(target_arch = "wasm32")]
-        let autosave = Subscription::none();
         // Drawings handed over by a second launch (single instance). Inert in a
         // process that lost the port election, so it costs nothing there.
         #[cfg(not(target_arch = "wasm32"))]

src/app/view/modal.rs

@@ -77,6 +77,8 @@ impl OpenCADStudio {
             Some(K::Recovery) => crate::tr!("modal", "recovery-report"),
             Some(K::MissingFonts) => crate::t!("Missing fonts").into_owned(),
             Some(K::RecoveryPrompt) => crate::tr!("modal", "recovery-prompt"),
+            #[cfg(target_arch = "wasm32")]
+            Some(K::AutosaveRestore) => crate::t!("Recover Drawing").into_owned(),
             Some(K::GpuWarning) => crate::tr!("gpu", "title"),
             Some(K::XrefHelp) => crate::t!("Reference Manager Help").into_owned(),
             Some(K::SheetSet) => self
@@ -1808,6 +1810,17 @@ impl OpenCADStudio {
                     crate::ui::window::recovery::view_prompt(&opening.name, error, flow)
                 })
             }
+            #[cfg(target_arch = "wasm32")]
+            super::super::ModalKind::AutosaveRestore => {
+                let name = self
+                    .web_autosave_pending
+                    .first()
+                    .map(|entry| entry.name.clone())
+                    .unwrap_or_default();
+                automatic_flow(ex, |flow| {
+                    autosave_restore_dialog_window(&name, flow)
+                })
+            }
         })
     }
 }
@@ -2055,6 +2068,40 @@ fn unsaved_changes_dialog_window(
     .into()
 }
 
+/// Web crash-recovery prompt: an autosave snapshot of `name` is newer than
+/// the drawing's last save. Restore opens it as a recovered tab; Discard
+/// deletes the snapshot. Mirrors the unsaved-changes dialog's layout.
+#[cfg(target_arch = "wasm32")]
+fn autosave_restore_dialog_window(
+    name: &str,
+    sizing: crate::ui::modal::ModalSizing,
+) -> Element<'static, Message> {
+    container(
+        column![
+            text(crate::tf!(
+                "\"{name}\" has an autosave copy newer than the last save."
+            ))
+            .size(14),
+            iced::widget::Space::new().height(4),
+            text(t!("Restore it as a recovered tab, or discard the copy."))
+                .size(12)
+                .style(dialog_muted_text_style),
+            iced::widget::Space::new().height(20),
+            row![
+                dialog_button(t!("Restore"), Message::WebAutosaveRestore, button::primary),
+                iced::widget::Space::new().width(8),
+                dialog_button(t!("Discard"), Message::WebAutosaveDiscard, button::danger),
+            ],
+        ]
+        .spacing(0),
+    )
+    .style(dialog_body_style)
+    .center_x(sizing.width)
+    .center_y(sizing.height)
+    .padding([24, 28])
+    .into()
+}
+
 #[cfg(not(target_arch = "wasm32"))]
 fn file_in_use_dialog_window(
     path: &str,