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/workbench/StatusBar.module.css

@@ -90,6 +90,12 @@
   text-overflow: ellipsis;
   white-space: nowrap;
 }
+.taskCancelOff {
+  display: inline-flex;
+}
+.taskCancelOff > button {
+  pointer-events: none;
+}
 .taskPct {
   font-family: var(--font-mono);
   font-size: var(--fs-small);