fix(web): preserve Benchmark selection after opening a Session trace

[zzzbitz]

Description

The Evaluation Center loses its selected Benchmark after navigating to a Session trace.

Reproduction

1. Open Evaluation Center and select an Agent and Benchmark.
2. Expand an evaluation, Case, and Run.
3. Click the Session link to open Trace Observability.
4. Click Evaluation Center in the sidebar.

The page returns to the empty default state instead of the previously selected Benchmark.

Expected

Restore the last selected Agent and Benchmark for the current Project when returning to Evaluation Center.

Code context

BenchmarkPage keeps selection only in component state. Navigating to the trace page unmounts it, while the sidebar returns to the bare /benchmark route. The existing agentId query parameter controls only which Agent group is expanded.

Observed on v0.2.0 at e05dea5.