FN-8229: remove background AI tasks pill

Replace the redundant footer AI session control with progress and action visibility in the session notification banner.

- Delete the BackgroundTasksIndicator component, styles, footer wiring, and obsolete translations.
- Surface non-planning generating and retained error sessions through the notification banner while leaving Planning in its dedicated view.
- Update session lifecycle, localization, documentation, and regression coverage.

Files changed:
 .../fn-8229-remove-ai-background-tasks-pill.md     |   7 +
 docs/dashboard-guide.md                            |   4 +-
 packages/dashboard/README.md                       |   1 -
 packages/dashboard/app/App.tsx                     |  28 +--
 .../app/__tests__/css-hygiene-scan.test.ts         |   2 -
 .../dashboard-component-color-tokenization.test.ts |   1 -
 packages/dashboard/app/api/legacy.ts               |   2 +-
 .../app/components/BackgroundTasksIndicator.css    | 130 -------------
 .../app/components/BackgroundTasksIndicator.tsx    | 210 ---------------------
 .../dashboard/app/components/ExecutorStatusBar.css |   3 -
 .../dashboard/app/components/ExecutorStatusBar.tsx |  30 +--
 .../app/components/SessionNotificationBanner.css   |  10 +
 .../app/components/SessionNotificationBanner.tsx   |  52 ++++-
 .../app/components/SubtaskBreakdownModal.tsx       |   7 +-
 .../app/components/__tests__/App.test.tsx          |  11 +-
 .../__tests__/ExecutorStatusBar.test.tsx           |   1 -
 .../__tests__/SessionNotificationBanner.test.tsx   |  88 ++++-----
 .../components/__tests__/utility-mobile.test.tsx   | 191 +------------------
 .../app/components/dashboard/DashboardBanners.tsx  |   8 +-
 .../dashboard/__tests__/DashboardBanners.test.tsx  |  30 ++-
 .../hooks/__tests__/useBackgroundSessions.test.ts  |  21 ++-
 .../dashboard/app/hooks/useBackgroundSessions.ts   |  21 ++-
 .../app/utils/__tests__/appLifecycle.test.ts       |  61 ++----
 packages/dashboard/app/utils/appLifecycle.ts       |  16 +-
 packages/dashboard/src/routes.ts                   |   2 +-
 packages/i18n/locales/en/app.json                  |  33 +---
 packages/i18n/locales/es/app.json                  |  44 ++---
 packages/i18n/locales/fr/app.json                  |  44 ++---
 packages/i18n/locales/ko/app.json                  |  44 ++---
 packages/i18n/locales/zh-CN/app.json               |  44 ++---
 packages/i18n/locales/zh-TW/app.json               |  44 ++---
 packages/i18n/src/resources.d.ts                   |  48 ++---
 32 files changed, 316 insertions(+), 922 deletions(-)

Fusion-Task-Id: FN-8229

Fusion-Task-Lineage: a232fb04-08ff-4f0a-a440-e94545243e61

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

packages/dashboard/app/api/legacy.ts

@@ -8420,7 +8420,7 @@ export function reorderTodoItems(listId: string, itemIds: string[], projectId?:
   });
 }
 
-// ── AI Sessions (Background Tasks) ─────────────────────────────────────────
+// ── AI Sessions ────────────────────────────────────────────────────────────
 
 /**
  * Needs-attention variants for a CLI agent session (CLI Agent Executor, U11).