fix(tasks): Make every background task cancellable, queued or running (#177)

Background tasks often couldn't be cancelled, or cancelling didn't stop them:
- A cancel that arrived before the backend registered the request was dropped (build while saving files, multi-repository fetch/pull between repositories). The host now remembers early cancels (bounded, 2 min), and request ids start at a random value per page.
- There was no queue and no common cancel path: backend-reported tasks (LSP progress, deployment transfers) couldn't be cancelled, and tool installs, VCS operations and update checks had no cancel button in Background Tasks.
- Only the direct child process was killed, and JDK / tool archives were extracted without checking for cancellation.

Tool installs and JDK downloads now share a downloads queue; cancelling a queued task removes it without starting any work. Every task cancels through one path that shows "Cancelling…" and then removes it; tasks that can't be interrupted show a disabled cancel button with the reason. IdeEvent::Task carries `cancellable` with a cancel_task command, cancellable LSP progress sends window/workDoneProgress/cancel, installers, curl and tar run in their own process group and are killed as a group, and extraction checks for cancellation between entries.

Claude-Session: https://claude.ai/code/session_01Dq6jQQn8ZhRfkkyj3PRutA

src/features/app/updates.ts

@@ -39,9 +39,13 @@ export function checkForUpdates(): Promise<void> {
 
 async function runCheck() {
   const title = t("app.update.checking");
-  upsertTask({ id: CHECK_TASK_ID, title, message: null, fraction: null, done: false });
+  // The check only reads: cancelling drops its answer (the request finishes on its own).
+  let abandon = () => {};
+  const abandoned = new Promise<"cancelled">((resolve) => (abandon = () => resolve("cancelled")));
+  upsertTask({ id: CHECK_TASK_ID, title, message: null, fraction: null, done: false, cancel: () => abandon() });
   try {
-    const r = await call(commands.appCheckUpdate());
+    const r = await Promise.race([call(commands.appCheckUpdate()), abandoned]);
+    if (r === "cancelled") return;
     if (!r.enabled) {
       notifications().notify({
         level: "info",
@@ -122,6 +126,8 @@ async function runInstall(version: string) {
       fraction,
       done: false,
       cancel: p.type === "installing" ? undefined : cancel,
+      // Replacing the app happens in one step that must not be cut short.
+      uncancellable: p.type === "installing" ? t("app.update.installUncancellable") : undefined,
     });
   };
   // The notification's action is spent: progress lives in the status bar from here.