[Dashboard] Investigate intermittent native-chart blank state

## Scope

Investigate whether the published Looker Studio report has a reproducible
intermittent **native-chart** blank state after page navigation.

The original custom/Vega diagnosis was incorrect. The canonical report uses
native Looker Studio bar, time-series, scorecard, and table components. A live
network pass across two complete render cycles observed zero requests to
`usercontent.goog` or any community-visualization resource.

Responsive/mobile support is separate product scope and is tracked in #386.

## Current evidence

The original blank-chart observation has not reproduced under the strengthened
protocol:

| Path | Observed result |
|---|---|
| Cold cache-miss load | Blank at 40 seconds; fully rendered by 70 seconds |
| Navigate away and return | Fully rendered within 10 seconds |
| SPA navigation to a second page | Fully rendered within 18 seconds |
| Network tracking | Zero community-visualization requests |

This evidence is an environment-specific baseline, not an SLA. It demonstrates
that page chrome, a loading indicator, or an empty chart container is not a
valid completion signal.

## Reproduction protocol

For a new rendering report:

1. Record browser/version, signed-in state, extension state, viewport, and
whether the load is cold or warm.
2. Run at least three fresh report loads.
3. In each run, visit all eight pages in order and repeat the complete
navigation loop three times.
4. Allow up to 90 seconds per page for non-degenerate chart output.
5. Capture timestamped page evidence, failed network requests, and matching
BigQuery job activity.
6. Verify the chart's native editor bindings before changing the report.

A defect is reproducible when the same chart remains blank after 90 seconds in
at least two fresh runs, or when the same navigation sequence reproduces the
failure in at least two loops. A legitimate expected empty state is not a
failure.

## Guardrails

- Do not classify a Google-owned render frame as evidence of Vega or a
community visualization.
- Do not replace chart types, bindings, filters, or SQL until the failure
reproduces under this protocol.
- Keep responsive/mobile layout work in #386.

## References

- #383
- #386
- `dashboard/looker_studio/docs/rendering-and-viewport-support.md`
- Live review evidence:
https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/pull/383#issuecomment-5098030747

Start with the published Looker Studio report and the reproduction protocol in the issue, recording browser, session, extension, viewport, load type, page evidence, failed requests, and matching BigQuery activity. Read dashboard/looker_studio/docs/rendering-and-viewport-support.md and verify native editor bindings before changing anything. Done means either reproducing the same blank chart under the stated thresholds or documenting that it does not reproduce with the collected evidence.

Written by the indexing model from the issue text.