All ~165 legacy var() usages (--color-*, --surface*, --accent*, --border,
--text*) with hex fallbacks and ~150 raw hex literals across app.scss and
24 scoped style blocks are swapped to the kit's --gn-* tokens per the
intent of the old fallback values:

- accent/primary/teal (#7aa2f7/#4ec9b0) -> --gn-color-secondary
- amber/orange highlights -> --gn-color-accent
- text greys -> --gn-color-text-{light,medium,dark}
- panels -> --gn-surface-panel; elevated tints -> color-mix over panel
- translucent overlays -> color-mix(in srgb, ..., transparent)

Deliberately literal: terminal deep bg #0f0f14 (darker than any token)
and the content-card image badge teal #89dceb (kit has no teal). File
type badges keep a categorical palette via nearest kit tokens.

Dead "UI kit overrides" block halved: kit 1.0 dropped hover icon
rotation, so only the app-owned chevron open-state flip remains.

Verified: vitest 111 passed, build OK, dev render matches W1 baseline

- new Backgrounds tab: task list (running first, badges, per-tool icons)
  with a detail view (status, timestamps, sub-agent tokens, result/progress)
- GET /sessions/{id}/tasks snapshot endpoint: task_update events are not
  replayed on reconnect, so the client fetches the task list on session
  load/reload; live task_update entries are merged on top (chat.fetchTasks)
- terminal task_update no longer removes the entry — it marks it finished
  so the tab shows recent completions; _terminalTaskIds still blocks
  resurrection by a late running update
- toasts for background task start / finish (info / success / error) in
  the WS dispatch; silent for other sessions and unknown terminal ids
- persistent background-tasks chip removed (replaced by the tab); only
  the queued-messages chip stays
- fix invisible status text in terminal detail rows: filled .status-*
  backgrounds now scope to .terminal-status-badge only