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/utils/labels.js

@@ -0,0 +1,6 @@
+import { MODEL_QUADRANTS } from '../hooks/useApi.js'
+
+/** model id -> display label, derived from the single source of truth in useApi.js. */
+export const MODEL_QUADRANT_LABEL = Object.fromEntries(
+  MODEL_QUADRANTS.map((q) => [q.model, q.label])
+)