ui: Embed flamegraph viewer in Heap Dump Explorer

Add a Flamegraph tab to the Heap Dump Explorer page that renders the
JAVA_HEAP_GRAPH flamegraph for the active dump using FlamegraphPanel.

The four metrics (Object Size / Object Count / Dominated Object Size /
Dominated Object Count) mirror the timeline-side flamegraph and are
parameterised on the active dump's (upid, ts) — switching dumps swaps
the data source. A single `buildMetric` helper assembles each metric
from the (table, value column, isDominator) triple it differs by, so
the four declarations stay short.

Flamegraph state (selected metric, filters, view) is held on
HeapDumpExplorerSession so it survives tab switches; reset on dump
change so the new dump opens with defaults rather than inheriting the
previous dump's filters. The setter is exposed as an arrow property
on the session and passed by reference into the FlamegraphView attrs.

Each node exposes a "Show objects from this class" action that opens a
new flamegraph-objects tab for the rows whose path_hash_stable matches
the clicked node, scoped to the active (upid, ts) — same flow used by
the timeline-side "Open in Heapdump Explorer" action. The page wires
the action to `session.openFlamegraph` directly.

A '#!/heapdump/flamegraph' subpage is added to NavState.

Change-Id: I3f55248ba09289574bd0f8aeb28c7d912f8f9951