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/page.test.tsx

@@ -3,6 +3,16 @@
 import { fireEvent, render, screen, waitFor } from "@testing-library/react";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
+// Mock showToast utility with vi.hoisted to avoid hoisting issues
+const mockShowToast = vi.hoisted(() => ({
+  success: vi.fn(),
+  error: vi.fn(),
+  info: vi.fn(),
+}));
+vi.mock("@/components/ui/toaster", () => ({
+  showToast: mockShowToast,
+}));
+
 // Mock fetch globally
 const mockFetch = vi.fn();
 global.fetch = mockFetch;
@@ -55,6 +65,9 @@ const mockUserResponse = {
 describe("Dashboard", () => {
   beforeEach(() => {
     vi.clearAllMocks();
+    mockShowToast.success.mockClear();
+    mockShowToast.error.mockClear();
+    mockShowToast.info.mockClear();
   });
 
   describe("rendering", () => {
@@ -496,6 +509,42 @@ describe("Dashboard", () => {
         expect(screen.getByText(/flare mode active/i)).toBeInTheDocument();
       });
     });
+
+    it("shows error toast when toggle fails", async () => {
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () => Promise.resolve(mockTodayResponse),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () => Promise.resolve(mockUserResponse),
+        });
+
+      render(<Dashboard />);
+
+      await waitFor(() => {
+        expect(screen.getByText("Flare Mode")).toBeInTheDocument();
+      });
+
+      // Clear mock and set up for failed toggle
+      mockFetch.mockClear();
+      mockFetch.mockResolvedValueOnce({
+        ok: false,
+        json: () => Promise.resolve({ error: "Failed to update override" }),
+      });
+
+      const flareCheckbox = screen.getByRole("checkbox", {
+        name: /flare mode/i,
+      });
+      fireEvent.click(flareCheckbox);
+
+      await waitFor(() => {
+        expect(mockShowToast.error).toHaveBeenCalledWith(
+          "Failed to update override",
+        );
+      });
+    });
   });
 
   describe("error handling", () => {