fix(insights): sleep-stage chart strokes resolve through Dracula tokens

CartesianGrid and axis labels were rendering un-themed because
`stroke="hsl(var(--border))"` evaluates to `hsl(#44475a)` —
invalid CSS — and the SVG fell back to currentColor. The Dracula
tokens are already hex, so the bare `var(--border)` /
`var(--muted-foreground)` form is the correct contract.

src/components/insights/sleep-stage-stacked-bar.tsx

@@ -244,19 +244,19 @@ export function SleepStageStackedBar({ breakdown }: SleepStageStackedBarProps) {
               >
                 <CartesianGrid
                   strokeDasharray="3 3"
-                  stroke="hsl(var(--border))"
+                  stroke="var(--border)"
                   opacity={0.3}
                 />
                 <XAxis
                   type="category"
                   dataKey="label"
-                  stroke="hsl(var(--muted-foreground))"
+                  stroke="var(--muted-foreground)"
                   fontSize={10}
                   interval="preserveStartEnd"
                 />
                 <YAxis
                   type="number"
-                  stroke="hsl(var(--muted-foreground))"
+                  stroke="var(--muted-foreground)"
                   fontSize={11}
                   tickFormatter={(v: number) => {
                     // Render the y-axis as hours so 480 min reads as 8h.