Add toast notification system with sonner library

- Create Toaster component wrapping sonner at bottom-right position
- Add showToast utility with success/error/info methods
- Error toasts persist until dismissed, others auto-dismiss after 5s
- Migrate error handling to toasts across all pages:
  - Dashboard (override toggle errors)
  - Settings (save/load success/error)
  - Garmin settings (connection success/error)
  - Calendar (load errors)
  - Period History (load/delete errors)
- Add dark mode support for toast styling
- Add Toaster provider to root layout
- 27 new tests (23 toaster component + 4 integration)
- Total: 977 unit tests passing

P5.2 COMPLETE - All P0-P5 items now complete.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/layout.tsx

@@ -1,8 +1,9 @@
 // ABOUTME: Root layout for PhaseFlow application.
-// ABOUTME: Configures fonts, metadata, and global styles.
+// ABOUTME: Configures fonts, metadata, Toaster provider, and global styles.
 import type { Metadata } from "next";
 import { Geist, Geist_Mono } from "next/font/google";
 import "./globals.css";
+import { Toaster } from "@/components/ui/toaster";
 
 const geistSans = Geist({
   variable: "--font-geist-sans",
@@ -37,6 +38,7 @@ export default function RootLayout({
           Skip to main content
         </a>
         {children}
+        <Toaster />
       </body>
     </html>
   );