feat: add hex map toggle for US congressional districts

[SakshiKekre]

Summary

• Add toggle to switch between geographic (natural boundaries) and hex grid views for congressional district maps
• Hex map shows each district as an equal-area shape, eliminating visual distortion where large rural districts dominate

Changes

New files

• congressional_districts_hex.geojson - Hex layout GeoJSON (based on Daily Kos/Downballot v3.1)
• MapTypeToggle.tsx - Toggle component for Geographic/Hex grid
• MapTypeToggle.test.tsx - Unit tests for toggle

Modified files

• USDistrictChoroplethMap.tsx - Support both visualization types
• utils.ts - Fix Plotly geo config for hex maps (no projection, fitbounds only)
• types.ts - Add MapVisualizationType and optional geo config properties
• AbsoluteChangeByDistrict.tsx / RelativeChangeByDistrict.tsx - Add toggle to pages
• README.md - Document 119th Congress compatibility and design decisions

Technical notes

Why irregular "blob" shapes instead of uniform hexagons?

The hex map uses irregular polygon shapes by design:

1. The artistic arrangement packs 436 districts into a recognizable US shape
2. In dense areas (NY, CA urban districts), some districts are only 0.5 units apart
3. Uniform hexagons (requiring ~1.0 unit spacing) would overlap
4. The blobs tessellate like puzzle pieces to avoid overlap

119th Congress compatibility

• All 436 district IDs match between hex map and 119th Congress natural boundaries
• The 5 states that redistricted (AL, GA, LA, NY, NC) kept the same number of districts
• Only PR-98 (Puerto Rico) is missing from hex map (non-voting delegate)

Test plan

• Toggle appears on congressional district pages
• Geographic view works as before
• Hex view renders correctly with proper colors
• Hover tooltips work on both views
• Toggle state persists while viewing the page

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