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>

src/components/ApproxNote.jsx

@@ -0,0 +1,10 @@
+import { DISTRIBUTION_APPROX_NOTE } from '../utils/distributionApprox.js'
+
+/** Consistent caption for any chart that renders an approximated distribution shape. */
+export default function ApproxNote() {
+  return (
+    <p style={{ fontSize: '0.78rem', fontStyle: 'italic', color: 'var(--cv-ink-3)', margin: '0.25rem 0 0' }}>
+      {DISTRIBUTION_APPROX_NOTE}
+    </p>
+  )
+}