fix(insights): stop the pulse chart from faking a continuous shape, add honest error states

Intraday pulse rendered sparse spot readings as a smooth interpolated
curve across hours-wide gaps, implying continuous monitoring the data
never had. The chart now grids one point per bucket, breaks the area
over any missing bucket (connectNulls={false}), and discloses coverage
("based on N readings across M hours") whenever that produces a visible
break.

Four surfaces also collapsed a failed fetch into the honest empty/no-
data state instead of surfacing the error: the Today hero, the
medication efficacy tab (previously stuck on the loading skeleton
forever on error), the intraday pulse chart, and the recent-workouts
dashboard tile. All four now branch on isError and render
QueryErrorCard with a retry action; the tile's "loading…" text line is
replaced with row-shaped skeletons.

src/hooks/use-workouts.ts

@@ -82,6 +82,10 @@ export interface UseWorkoutsResult {
   isLoading: boolean;
   isEmpty: boolean;
   error: Error | null;
+  /** True once the query has exhausted retries and settled on a failure. */
+  isError: boolean;
+  /** Re-runs the query — the retry action for a failed fetch. */
+  refetch: () => void;
 }
 
 export function useWorkouts(opts: UseWorkoutsOptions = {}): UseWorkoutsResult {
@@ -109,6 +113,8 @@ export function useWorkouts(opts: UseWorkoutsOptions = {}): UseWorkoutsResult {
     isLoading: query.isLoading,
     isEmpty,
     error: query.error as Error | null,
+    isError: query.isError,
+    refetch: () => void query.refetch(),
   };
 }