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/tools/__tests__/tools.test.tsx

@@ -70,6 +70,22 @@ describe("tools store", () => {
     expect(useTools.getState().tools.find((t) => t.name === "gopls")?.status).toBe("missing");
   });
 
+  it("installs one at a time; one cancelled while it waits never starts", async () => {
+    backend.stepMs = 10;
+    await refreshTools();
+    const install = vi.spyOn(backend, "install");
+    const first = installTool("gopls");
+    const queued = installTool("vtsls");
+    expect(useTasks.getState().tasks.find((t) => t.id === "tools:vtsls")).toMatchObject({ queued: true });
+    cancelInstall("vtsls");
+    expect(useTasks.getState().tasks.find((t) => t.id === "tools:vtsls")).toBeUndefined();
+    expect(await queued).toBeNull();
+    expect(useTools.getState().logs["vtsls"]?.error).toBe("Cancelled");
+    expect((await first)?.status).toBe("installed");
+    expect(install.mock.calls.map((c) => c[0])).toEqual(["gopls"]);
+    expect(notes.filter((n) => n.level === "error")).toHaveLength(0);
+  });
+
   it("uninstalls", async () => {
     await refreshTools();
     expect(await uninstallTool("rust-analyzer")).toBe(true);