Support collapsible widget sections in dashboards

### Problem Statement

Dashboards with many widgets become visually overwhelming and difficult to parse at a glance. When a dashboard monitors multiple similar entities (e.g. critical routes, services, or environments), the widget layout becomes too long to scan quickly — users have no way to collapse groups of related widgets or visually separate logical sections.

**Current behavior:** All widgets render in a flat grid. The only organizational tools are widget titles and manual positioning.

**Gap:** There is no concept of named, collapsible sections within a single dashboard. Users who want to group related widgets — and optionally hide them to reduce visual noise — have no supported path to do so.

### Proposed Solution

Allow users to create named sections within a dashboard that:
- Group a set of widgets under a labeled header
- Can be collapsed/expanded to reduce visual noise
- Persist their collapsed/expanded state (per-user or per-dashboard)

### Product Area

Dashboards

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No files, tests, or implementation entry points are named. Start by locating the dashboard widget layout and the existing dashboard state or persistence paths; the work is done when named widget sections can collapse and expand and their state persists per the chosen scope.

Written by the indexing model from the issue text.