[Dashboard] Evaluate a responsive Looker Studio template

## Context

The published BigQuery Agent Analytics dashboard is a multi-component,
freeform Looker Studio report. Version 1 intentionally targets desktop
viewports:

- minimum supported width: 1280 CSS pixels;
- recommended width: 1440 CSS pixels; and
- latest live validation width: 1568 CSS pixels.

At phone and narrow-tablet widths, the fixed canvas requires horizontal
scrolling and the navigation and report controls are difficult to use.

This is separate from the native-chart rendering investigation in #381.
PR #383 documented that the current charts are native Looker Studio charts and
that mobile support cannot be solved by changing chart implementations.

## Product decision

Choose and document one of these support models:

1. **Desktop-only v1** — retain the freeform report, validate the 1280-pixel
boundary, and document desktop display guidance.
2. **Mobile-supported v2** — build and maintain a separately owned responsive
Looker Studio report.

The current pages contain multiple components. They cannot be converted in
place without rebuilding their layout, so a responsive report is a new
template rather than a modification of the reviewed freeform template.

## Acceptance criteria for a responsive template

- [ ] Maintainers explicitly approve mobile/tablet support and template
ownership.
- [ ] Build a separate responsive report; do not mutate the canonical freeform
template in place.
- [ ] Preserve the three-identifier hydration flow.
- [ ] Verify Viewer's Credentials before sharing.
- [ ] Re-run all 37 chart semantic checks.
- [ ] Re-run query-cost and date-window validation.
- [ ] Validate representative phone, narrow-tablet, and desktop viewports.
- [ ] Publish separate support and maintenance expectations for both templates.

## References

- #381
- #383
- `dashboard/looker_studio/docs/rendering-and-viewport-support.md`
- https://docs.cloud.google.com/data-studio/create-a-responsive-report

Start with dashboard/looker_studio/docs/rendering-and-viewport-support.md and review #381 and #383 for the existing rendering and viewport constraints. Confirm the maintainer decision on support model and ownership before evaluating or building anything. Done means the approved model is documented, and any responsive template meets the listed hydration, credential, semantic-check, cost, date-window, viewport, and maintenance criteria.

Written by the indexing model from the issue text.