Add CAP alert area intersection validation

[erick-otenyo]

Add CAP alert area intersection validation

Overview

Implements real-time and server-side validation to prevent overlapping alert areas, ensuring spatial consistency and avoiding ambiguity in alert dissemination.

Changes

Frontend

• Added AreaRegistry class (area-registry.js) that manages shared state across all area widgets on the same page
• Each area widget (PolygonWidget, MultiPolygonWidget, BoundaryPolygonWidget, CircleWidget) registers itself with AreaRegistry on initialization
• Sibling areas from other blocks are displayed in the background of each map widget as a grayed-out overlay with a dashed outline, providing spatial context while drawing
• Real-time intersection detection runs on every geometry change using turf.booleanIntersects and turf.intersect
• Intersection check uses an area threshold (default 1000m²) to allow adjacent areas that share administrative boundaries without flagging them as intersecting
• MutationObserver watches for block deletion via Wagtail's aria-hidden pattern and cleans up registry state automatically
• Config endpoint is fetched once and cached on window.alertAreaConfig to avoid redundant requests across multiple widgets
• window.alertAreasHasIntersection provides a global flag that can be used to block further actions while intersections exist

Backend

• Added intersection check in AlertInfo.clean using shapely
• Intersection area is computed in m² using a degrees-to-metres approximation suitable for equatorial/African latitudes
• Same 1000m² threshold applied server-side as a fallback, overridable per site
• intersection_area_threshold added to CapSetting model as a configurable field
• Threshold is passed down to AlertInfo block from CapAlertPageForm.__init__ where site context is available

Self-intersection handling

• Single polygon self-intersections (bowtie/figure-8 shapes) are detected using turf.kinks
• On draw.create: invalid polygon is deleted and user is prompted to redraw
• On save: commit is blocked and user stays in edit mode until fixed
• Validation is centralised in setDrawData which returns false on failure

Notes

• The 1000m² threshold accounts for floating point imprecision along shared administrative boundaries — two adjacent areas that share a border will not be flagged as intersecting
• The threshold is configurable per site via CapSetting in the Wagtail admin, allowing NMHSs with imprecise boundary data to adjust accordingly
• Server-side and client-side thresholds are kept in sync via the map widget config endpoint

[erick-otenyo]

Closes #122

Closes #67