Performance Benchmarking - Typical Traffic

This epic adds a second tab, Typical Traffic, to the `analyticsTrafficOverview` widget on the main dashboard, behind a new `typicalTraffic` feature flag: thirteen months of the site's own daily traffic in one chart, and a section naming the factors that moved traffic in the selected period — together with the server-side `GET:benchmarking-data` datapoint on `Analytics_4` that gathers and derives everything the tab draws.

Link to the design doc: https://docs.google.com/document/d/1dsEs6-NjlP_LNqz5md5fnJMuxh9Vd9f88w4DTrZdLok/edit?tab=t.y7e2u5h52vf1

Start with the analyticsTrafficOverview widget and the server-side GET:benchmarking-data datapoint on Analytics_4, using the linked design document to understand the Typical Traffic tab. The work is complete when the feature-flagged tab shows thirteen months of daily traffic and the selected-period traffic factors supplied by the datapoint.

Written by the indexing model from the issue text.