FN-6040: show loading indicators for task detail activity and agent logs

Add first-load loading states so task activity and agent log views do not flash empty states.

- show a loading status row in the task detail Activity tab while detail history is still loading
- keep agent logs in a loading state until the active task/project context has completed its initial fetch
- add accessible loading markup, regression tests, and dashboard guide documentation for the new behavior

Files changed:
 docs/dashboard-guide.md                            |   1 +
 .../dashboard/app/components/AgentLogViewer.css    |  18 +++
 .../dashboard/app/components/AgentLogViewer.tsx    |   2 +-
 .../dashboard/app/components/TaskDetailModal.css   |  17 +++
 .../dashboard/app/components/TaskDetailModal.tsx   |   7 +-
 .../components/__tests__/AgentLogViewer.test.tsx   |   3 +-
 .../components/__tests__/TaskDetailModal.test.tsx  | 144 +++++++++++++++++++++
 .../app/hooks/__tests__/useAgentLogs.test.ts       |  74 +++++++++++
 packages/dashboard/app/hooks/useAgentLogs.ts       |  14 +-
 9 files changed, 276 insertions(+), 4 deletions(-)

Fusion-Task-Id: FN-6040

Fusion-Task-Lineage: cd4b8f7f-63cd-4d75-be00-d362e915f4e8

docs/dashboard-guide.md

@@ -661,6 +661,7 @@ The **Logs** tab includes an **Agent Log** subview designed for debugging long-r
 
 - Full `thinking`, `tool_result`, and `tool_error` payloads are shown without entry-content truncation.
 - Raw tool output is rendered as multiline blocks, preserving line breaks and indentation.
+- The Activity and Agent Log subviews show loading indicators while their first async history/detail request is pending, so empty states only appear after the relevant fetch completes.
 - The initial load fetches a recent page, then **Load More** progressively prepends older history.
 - Live streaming appends new entries in chronological order while preserving your scroll position when loading older pages.
 - The **Markdown / Plain** toggle lets you switch between formatted markdown and literal/raw text rendering.