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>

package.json

@@ -29,6 +29,7 @@
     "react": "19.2.3",
     "react-dom": "19.2.3",
     "resend": "^6.7.0",
+    "sonner": "^2.0.7",
     "tailwind-merge": "^3.4.0",
     "zod": "^4.3.5"
   },
@@ -39,6 +40,7 @@
     "@testing-library/dom": "^10.4.1",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.1",
+    "@testing-library/user-event": "^14.6.1",
     "@types/node": "^20",
     "@types/node-cron": "^3.0.11",
     "@types/react": "^19",