FN-5845: fix mobile agent log scroll and add collapsible branch group card

Improve dashboard mobile log usability and make branch group cards collapsible for cleaner review workflows.

- preserve/restore AgentLogViewer scroll position when toggling mobile drawer and lock body scroll while open
- add collapse/expand state to BranchGroupCard with a summary header and hidden details when collapsed
- add regression tests for AgentLogViewer mobile scroll behavior and BranchGroupCard collapse interactions
- add a changeset and dashboard guide note for the new branch group card behavior

Files changed:
 .changeset/fn-5845-branch-group-collapse.md        |  5 ++
 docs/dashboard-guide.md                            |  1 +
 .../dashboard/app/components/AgentLogViewer.tsx    | 21 ++++++++
 .../dashboard/app/components/BranchGroupCard.css   | 23 ++++++++
 .../dashboard/app/components/BranchGroupCard.tsx   | 63 +++++++++++++++++-----
 .../components/__tests__/AgentLogViewer.test.tsx   | 48 +++++++++++++++++
 .../components/__tests__/BranchGroupCard.test.tsx  | 18 +++++++
 7 files changed, 166 insertions(+), 13 deletions(-)

Fusion-Task-Id: FN-5845

Fusion-Task-Lineage: 6a0f2967-5df8-4761-ae6c-50991f01ab78

docs/dashboard-guide.md

@@ -1135,6 +1135,7 @@ UI surfaces:
 - Task cards show grouped/shared branch metadata for grouped tasks.
 - Clicking either grouped badge opens the dedicated **Group Task Modal** for that branch group.
 - Task detail renders a branch-group card with member landed progress.
+- In Task Detail Logs on mobile, the branch-group card includes a collapse/expand toggle so logs can reclaim vertical space while keeping group summary progress visible.
 
 The Group Task Modal shows shared branch name/status, member list (`taskId`, title, column, landed state), quick links to open each member task detail, completion progress (`X of Y members finished`), and tracked PR state when present. It live-refreshes from the same dashboard task-update stream and ignores stale cross-project events.