Redesign chart visualizations to match Civilytics white-paper style

Rebuilds all 6 charts around a validated categorical race palette, row-based
sex encoding, and a consistent observed-vs-modeled mark convention (diamond
vs. filled bar/density) instead of ad hoc per-chart color schemes. Adds a
shared student-group filter (defaults to all 8 groups) that scopes every
chart's data from one place in ChartPanel.

Drops the D3 dependency entirely in favor of plain SVG, removing the
imperative-DOM bug class behind this app's repeated "fix the fix" commits.
Replaces the fake symmetric-normal posterior approximation with a skewed,
median-preserving fit to the API's interval bounds, clearly labeled as an
approximation. Fixes two broken SVG fill attributes, a decorative model
dropdown that never affected its chart, dead code, an orphaned component,
and a broken CSS token reference.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -25,8 +25,5 @@
     "react-dom": "^19.2.8",
     "vite": "^8.2.1"
   },
-  "type": "module",
-  "dependencies": {
-    "d3": "^7.9.0"
-  }
+  "type": "module"
 }