(viewer) Move flamegraph into sidebar instead of full-screen overlay (#291)

* feat(viewer): move flamegraph into sidebar instead of full-screen overlay

The full-screen overlay was disruptive -- clicking a flamegraph or
viewing blocking calls would hide the entire timeline, making it
impossible to correlate profiling data with what was happening on
the workers. By rendering the flamegraph in the sidebar, the timeline
stays visible and interactive while inspecting CPU profiles.

The pop-out button is preserved for opening in a dedicated tab. Escape
still clears the flamegraph (after letting the flamegraph module handle
search/zoom reset first).

* feat(viewer): move blocking calls panel into sidebar

Same reasoning as the flamegraph change -- the full-screen overlay hid
the timeline, making it impossible to correlate blocking call data with
what was happening on the workers. The group-by dropdown is now rendered
inline in the sidebar body, and jump-to-poll links close the sidebar
before zooming to the target poll.

* feat(viewer): add tabs to switch between flamegraph and blocking calls

Replace the one-way selection picker dialog with tab-style toggles in
the sidebar header. When a region has both CPU samples and sched events,
tabs appear allowing instant switching between views without reselecting.
The selection range is preserved across tab switches.

* feat(viewer): add Poll Detail tab label for single-poll stack popup

The sidebar now shows a 'Poll Detail' tab when clicking a poll bar or
red spot, making it visually distinct from the Flamegraph and Blocking
Calls modes. Previously the stack popup had no tab indicator, which was
confusing when it replaced flamegraph content in the same sidebar.

Author: zz85