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/sonner.tsx

@@ -1,10 +1,14 @@
 "use client";
 
 import { Toaster as Sonner, type ToasterProps } from "sonner";
+import { useTheme } from "~/components/providers/theme-provider";
 
 const Toaster = ({ ...props }: ToasterProps) => {
+  const { theme = "system" } = useTheme();
+
   return (
     <Sonner
+      theme={theme as ToasterProps["theme"]}
       className="toaster group"
       position="bottom-right"
       closeButton