docs: describe the task panel that replaces sync UI - #146

Documents rotki/rotki#13148, which makes the task dock the one place background work is reported.

What changed

• Utilities, Background Tasks: replaces the notifications-tray task list with the task panel: the pill, the job rows and what a job lists when opened (icons, address links, the account's query and range, decode counts and protocol caches, per-section counts), the sync hint, what stays after a job finishes, grouped failures with their retry, dismissing, and what Stop all interrupts. The heading and its anchor are unchanged, since /usage-guides/long-running-tasks redirects to it.
• History events, Refreshing Events: points to the task panel instead of the removed query status panel.
• Dashboard: the top bar now only says when the history needs a refresh (never downloaded, stale, not fully decoded, just updated); progress moved to the task panel.

Screenshots

The four screenshots of the removed UI (query_status_button, query_status, pending_tasks, pending_tasks_cancel) are deleted rather than left showing UI that no longer exists. New captures of the panel are owed from the demo rig once #13148 merges.

Checks

pnpm run lint, check:images, check:anchors and check:app-links pass.