feat(maestro): force-kill prompt in the web client

The kill button now sends the unforced DELETE immediately instead of
asking first. Only the daemon's guard refusal raises a prompt, and it is
a real dialog rather than confirm(), so it can offer the override the
daemon already implements.

The API error envelope is decoded into ApiRequestError, keeping the code
and the unsaved-work breakdown so the dialog names what is in the way.

web/src/api.ts

@@ -1,6 +1,7 @@
 import * as routes from "./routes.js";
 import type {
   ApiError,
+  InTheWay,
   Branches,
   CreateSessionRequest,
   DaemonInfo,
@@ -13,19 +14,38 @@ import type {
   Uuid,
 } from "./types.js";
 
+/** A `/api/*` failure with the daemon's own error envelope kept intact, so a
+ * caller can branch on `code` — the kill guard's `kill_guarded` (#79) — and
+ * render `unsavedWork` rather than re-parsing the message. */
+export class ApiRequestError extends Error {
+  readonly status: number;
+  readonly code: string;
+  readonly detail: string;
+  readonly unsavedWork: InTheWay | null;
+
+  constructor(res: Response, info?: ApiError["error"], text?: string) {
+    const detail = info ? `${info.code}: ${info.message}` : (text ?? "");
+    super(`${res.status} ${res.statusText}${detail ? ` — ${detail}` : ""}`);
+    this.name = "ApiRequestError";
+    this.status = res.status;
+    this.code = info?.code ?? "";
+    this.detail = info?.message ?? text ?? "";
+    this.unsavedWork = info?.unsaved_work ?? null;
+  }
+}
+
 async function jsonOrError<T>(res: Response): Promise<T> {
   if (res.ok) {
     if (res.status === 204) return undefined as unknown as T;
     return (await res.json()) as T;
   }
-  let detail = "";
   try {
     const body = (await res.json()) as ApiError;
-    detail = `${body.error.code}: ${body.error.message}`;
-  } catch {
-    detail = await res.text();
+    throw new ApiRequestError(res, body.error);
+  } catch (e) {
+    if (e instanceof ApiRequestError) throw e;
+    throw new ApiRequestError(res, undefined, await res.text().catch(() => ""));
   }
-  throw new Error(`${res.status} ${res.statusText}${detail ? ` — ${detail}` : ""}`);
 }
 
 export async function listSessions(): Promise<Session[]> {
@@ -57,9 +77,14 @@ export async function renameSession(id: Uuid, name: string): Promise<Session> {
   );
 }
 
-export async function killSession(id: Uuid): Promise<void> {
+/** Kill a session. With `force` (#81) the daemon overrides its refusal over a
+ * worktree holding unsaved work and leaves that worktree on disk. */
+export async function killSession(id: Uuid, force = false): Promise<void> {
+  const url = force
+    ? `${routes.session(id)}?${routes.FORCE_QUERY}=true`
+    : routes.session(id);
   await jsonOrError(
-    await fetch(routes.session(id), {
+    await fetch(url, {
       method: "DELETE",
       credentials: "same-origin",
     }),