FIX: Collapse scenario atomic attack groups by default

[Roman Lutz (romanlutz)]

Description

Large scenario runs can display thousands of atomic group headings, pushing the rest of the results far down the page even when each group's details are collapsed.

Collapse the entire Atomic attack groups section by default on the shared active-run and saved-results page. Keep the group and execution counts visible beside an accessible Fluent UI Expand/Collapse control, and only mount the list when expanded. The user's choice survives progress updates for the same run and resets when opening another run.

Existing group metrics, execution details, conversation links, and actions remain available. Individual group expansion choices survive collapsing and reopening the section. This does not change the existing per-group execution limit or add pagination, virtualization, or a broader redesign.

Before (left) and after (right): the same synthetic run with 1,716 groups and 10,296 executions.

Tests and Documentation

• Added regressions for collapsed defaults on active and completed runs, visible counts, no hidden list mounting, keyboard expand/collapse, polling/rerender stability, switching runs, empty states, and access to every group and the last group's details.
• Passed 44 tests across ScenarioRunPage.test.tsx, ScenarioFlow.test.tsx, and useScenarioRunProgress.test.tsx.
• Passed frontend type-check, lint, and production build.
• Verified desktop/mobile behavior in an isolated browser preview using mocked 1,716-group / 10,296-execution data, including polling, run switching, keyboard controls, and the last group's conversation link. No scans or model requests were made.
• Updated doc/gui/0_gui.md to explain the disclosure behavior.

The local large-file commit hook failed in its git check-attr invocation. All three staged file sizes were verified below the configured 3 MB limit, then only that hook was skipped for the commit. Other applicable commit hooks passed.

JupyText: N/A; no notebooks or Python examples changed.