feat: adopt ShadCN chart primitives

Add ChartContainer, ChartConfig, ChartTooltip, ChartTooltipContent
primitives ported from ShadCN's chart pattern. Refactor all 3 chart
locations (dashboard, funnels, uptime) to use CSS variable-driven
theming instead of duplicated CHART_COLORS_LIGHT/DARK objects.

- Add --chart-1 through --chart-5, --chart-grid, --chart-axis CSS vars
- Remove duplicated color objects from 3 files (-223 lines)
- Add accessibilityLayer to all charts
- Rounded bar corners on funnel chart
- Tooltips use Tailwind dark classes instead of imperative style props

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

styles/globals.css

@@ -8,6 +8,25 @@
     --color-success: #10B981;
     --color-warning: #F59E0B;
     --color-error: #EF4444;
+
+    /* * Chart colors */
+    --chart-1: #FD5E0F;
+    --chart-2: #3b82f6;
+    --chart-3: #22c55e;
+    --chart-4: #a855f7;
+    --chart-5: #f59e0b;
+    --chart-grid: #f5f5f5;
+    --chart-axis: #a3a3a3;
+  }
+
+  .dark {
+    --chart-1: #FD5E0F;
+    --chart-2: #60a5fa;
+    --chart-3: #4ade80;
+    --chart-4: #c084fc;
+    --chart-5: #fbbf24;
+    --chart-grid: #262626;
+    --chart-axis: #737373;
   }
   
   body {