fix(begin): read sp-textfield value from DOM to work around Dioxus serialization

Dioxus's serializeInputEvent only reads event.target.value for native
HTMLInputElement/HTMLTextAreaElement/HTMLSelectElement. Custom elements
(sp-textfield) always serialize as "". Previously, e.value() returned ""
which both failed to update the sheet and triggered a re-render that
reset the DOM value back to "", causing typed input to disappear.

Fix: ignore the serialized value; spawn an async task that reads the
correct value from the DOM via document::eval after sp-textfield has
updated its own value property.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sean-parent

begin/src/inspector.rs

@@ -71,21 +71,34 @@ fn CellRow(id: CellId, sheet: Signal<Sheet>, labels: Signal<Labels>) -> Element
                 id: field_id,
                 value: input.read().clone(),
                 invalid: *has_error.read(),
-                oninput: move |e: FormEvent| {
-                    let s = e.value();
-                    input.set(s.clone());
-                    let mut sheet_w = sheet.write();
-                    let labels_r = labels.read();
-                    if let Some(meta) = labels_r.cells.get(&id)
-                        && (meta.write_str)(&mut sheet_w, &s).is_ok()
-                    {
-                        let result = if sheet_w.is_source(id) {
-                            sheet_w.propagate_without_replan()
-                        } else {
-                            sheet_w.propagate()
-                        };
-                        has_error.set(result.is_err());
-                    }
+                // Dioxus's event serializer only reads `event.target.value` for
+                // native HTMLInputElement — custom elements always serialize as "".
+                // Read the DOM value directly after the event so sp-textfield has
+                // updated its own value property.
+                oninput: move |_: FormEvent| {
+                    spawn(async move {
+                        let val = document::eval(&format!(
+                            r#"document.getElementById("cell-{id:?}").value"#
+                        ))
+                        .await
+                        .ok()
+                        .and_then(|v| v.as_str().map(String::from))
+                        .unwrap_or_default();
+
+                        input.set(val.clone());
+                        let mut sheet_w = sheet.write();
+                        let labels_r = labels.read();
+                        if let Some(meta) = labels_r.cells.get(&id)
+                            && (meta.write_str)(&mut sheet_w, &val).is_ok()
+                        {
+                            let result = if sheet_w.is_source(id) {
+                                sheet_w.propagate_without_replan()
+                            } else {
+                                sheet_w.propagate()
+                            };
+                            has_error.set(result.is_err());
+                        }
+                    });
                 },
                 onfocus: move |_| is_focused.set(true),
                 onblur: move |_| {