fix(insights): lift explainer popover body to 12px + correct target-summary doc

Two small polish items on the insights category surface.

The desktop explainer popover body rendered at 11px while the mobile
sheet used the larger `text-sm`, an uncomfortable and asymmetric read.
Lift the popover body to `text-xs` (12px) so both surfaces sit on the
type scale.

The `MetricTargetSummary` docstring claimed both the range and the
in-target share are hidden when the route flags `insufficientData`. The
range stays visible — only the share is suppressed. Correct the comment
to match the code.

src/components/insights/metric-explainer.tsx

@@ -113,7 +113,7 @@ export function MetricExplainer({ metric, className }: MetricExplainerProps) {
         <p
           id={bodyId}
           data-slot="metric-explainer-body"
-          className="text-muted-foreground text-[11px] leading-snug"
+          className="text-muted-foreground text-xs leading-snug"
         >
           {body}
         </p>

src/components/insights/metric-target-summary.tsx

@@ -29,9 +29,10 @@ import { Card, CardContent } from "@/components/ui/card";
  *
  * The "in target" share reads `daysInRange30d / daysLogged30d` — the
  * fraction of logged days over the last 30 whose mean reading landed in
- * the green band. We hide the share (and the range) entirely when the
- * route flags `insufficientData`, matching the `/targets` consistency
- * strip so the two surfaces never disagree.
+ * the green band. We hide the share alone when the route flags
+ * `insufficientData`; the range stays visible so the band is always
+ * legible. This matches the `/targets` consistency strip so the two
+ * surfaces never disagree.
  *
  * A discreet "Adjust target range" link routes to `/targets`, which
  * stays the canonical place to view / tune the bands.