feat(assets): Add platformized assets page with dashboard migration flag

[DominikB2014]

Adds a platformized assets landing page that conditionally renders the FRONTEND_ASSETS prebuilt dashboard when the insights-frontend-assets-dashboard-migration feature flag is enabled.

When the flag is off, the existing ResourcesLandingPage renders as before. When the flag is on, users see the prebuilt dashboard via PrebuiltDashboardRenderer.

Follows the same pattern used by web vitals (useHasDashboardsPlatformizedWebVitals + PlatformizedWebVitalsOverview) and other platformized insight pages.

Also fixes the GlobalFilter.value format for the span.op filter in the FRONTEND_ASSETS prebuilt config — it was missing the tagKey: prefix required by the filter condition string format.

Refs BROWSE-361

[linear]

BROWSE-361 Scaffold basic assets module

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Third row widgets overlap second row at same y-position

Medium Severity

THIRD_ROW_WIDGETS is placed at y: 1, which is the same y-position as SECOND_ROW_WIDGETS. Since SECOND_ROW_WIDGETS occupies y: 1 with h: 1, the third row needs to start at y: 2 to avoid overlapping. The variable naming ("SECOND_ROW" vs "THIRD_ROW") clearly indicates these are meant to be separate rows. While the dashboard layout engine may auto-resolve the overlap by pushing widgets down, specifying the correct position avoids relying on that behavior and keeps layout intent clear.