refactor: improve invoice editor UX and fix visual issues

- Remove clock icons and hour text from calendar month view, show only activity bars
- Fix calendar week view mobile layout (2-column grid instead of vertical stack)
- Update invoice form skeleton to match actual layout structure
- Add client-side validation for empty invoice item descriptions with auto-scroll to error
- Fix hourly rate defaulting logic with proper type guards
- Update invoice details skeleton to match page structure with PageHeader
- Fix hydration error in sidebar (div inside button -> span)
- Improve dashboard chart color consistency (draft status now matches monthly metrics)
- Fix mobile header layout to prevent text squishing (vertical stack on mobile)
- Add IDs to invoice line items for scroll-into-view functionality

src/components/ui/card.tsx

@@ -7,7 +7,7 @@ function Card({ className, ...props }: React.ComponentProps<"div">) {
     <div
       data-slot="card"
       className={cn(
-        "bg-card text-card-foreground border-border/40 flex flex-col rounded-lg border shadow-lg",
+        "bg-background/80 backdrop-blur-xl border-border/50 text-card-foreground flex flex-col rounded-3xl border shadow-sm overflow-hidden",
         className,
       )}
       {...props}