feat(maestro): spawn form back in a modal on web

Draft panes are gone: an empty pane is a placeholder again, and every
spawn affordance opens a dialog around the shared spawn form. Escape,
Cancel, the ✕ and a backdrop click dismiss it; a successful submit puts
the session in a new tab (Spawn similar and the fast-forward fallback
keep placing it beside the pane they acted on).

Removes the per-leaf draft state, its persistence strip, and the form
reuse/teardown machinery the panes needed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/src/spawnModal.ts

@@ -0,0 +1,96 @@
+// The spawn modal: dialog chrome around the shared spawn form (spawnForm.ts).
+//
+// One modal at a time — opening while one is up replaces it. Escape, the ✕ and
+// a click on the backdrop all close it; the form's own Escape handlers (the
+// directory autocomplete) swallow the key first, so dismissing a dropdown never
+// also discards the form.
+
+import { createSpawnForm, type SpawnFormSeed } from "./spawnForm.js";
+import type { Session } from "./types.js";
+
+/** Seed values for the spawn form — used by "Spawn similar" to open the dialog
+ * with the same cwd, command and name as an existing session. */
+export type SpawnPrefill = SpawnFormSeed;
+
+export interface SpawnModalOptions {
+  seed?: SpawnPrefill;
+  /** Called with the created session after a successful submit; the modal has
+   *  already closed by then, so the caller decides where it lands. */
+  onSuccess: (session: Session) => void;
+}
+
+/** Closer for the modal currently up, so opening a second one tears the first
+ *  one down properly — dropping its node alone would strand the form's global
+ *  listeners (dir autocomplete) and the Escape handler. */
+let closeCurrent: (() => void) | null = null;
+
+export function openSpawnModal(opts: SpawnModalOptions): void {
+  closeSpawnModal();
+
+  const host = document.createElement("div");
+  host.id = "modal-host";
+  host.className = "modal-backdrop";
+  const dialog = document.createElement("div");
+  dialog.className = "modal spawn-modal";
+  dialog.setAttribute("role", "dialog");
+  dialog.setAttribute("aria-modal", "true");
+  dialog.setAttribute("aria-labelledby", "spawn-title");
+  const header = document.createElement("header");
+  header.className = "modal-header";
+  const title = document.createElement("h2");
+  title.id = "spawn-title";
+  // "Spawn similar" arrives with a seed; the title says which act this is.
+  title.textContent = opts.seed ? "Spawn similar" : "Spawn session";
+  const closeBtn = document.createElement("button");
+  closeBtn.type = "button";
+  closeBtn.className = "btn btn-icon close";
+  closeBtn.setAttribute("aria-label", "Close");
+  closeBtn.textContent = "✕";
+  header.append(title, closeBtn);
+  dialog.appendChild(header);
+  host.appendChild(dialog);
+  document.body.appendChild(host);
+
+  const form = createSpawnForm({
+    seed: opts.seed,
+    onSuccess: (session) => {
+      close();
+      opts.onSuccess(session);
+    },
+  });
+  dialog.appendChild(form.el);
+
+  function close(): void {
+    if (closeCurrent === close) closeCurrent = null;
+    window.removeEventListener("keydown", onKeydown);
+    form.destroy();
+    host.remove();
+  }
+  closeCurrent = close;
+
+  function onKeydown(e: KeyboardEvent): void {
+    if (e.key === "Escape") {
+      e.preventDefault();
+      close();
+    }
+  }
+
+  // The modal keeps its Cancel affordance; the shared form ships only Submit.
+  const cancel = document.createElement("button");
+  cancel.type = "button";
+  cancel.className = "btn btn-ghost cancel";
+  cancel.textContent = "Cancel";
+  cancel.addEventListener("click", close);
+  form.el.querySelector(".form-actions")?.prepend(cancel);
+
+  closeBtn.addEventListener("click", close);
+  host.addEventListener("click", (e) => {
+    if (e.target === host) close();
+  });
+  window.addEventListener("keydown", onKeydown);
+}
+
+/** Close whatever spawn modal is up. No-op when none is. */
+export function closeSpawnModal(): void {
+  closeCurrent?.();
+}