fix(frontend): handle dashboard share clipboard failures (JhaSourav07#3037)

## Description

Fixes JhaSourav07#3036

## Pillar

- [x] 🛠️ Other (Bug fix, refactoring, docs)

## What changed

The dashboard Share button was calling
`navigator.clipboard.writeText(window.location.href)` without awaiting
the returned promise, then immediately showing `Link copied to
clipboard!`.

That meant browsers could reject the clipboard write while the UI still
reported success. I changed that handler to await the clipboard write,
show the success toast only after it resolves, and show an error toast
when copying fails.

I also added focused `DashboardClient` tests for both outcomes:

- successful dashboard link copy
- rejected clipboard write due to permission/browser failure

Opened as part of GSSoC 2026.

## Visual Preview

N/A — this is a dashboard interaction/error-handling fix.

## Validation

- [x] `npm run format`
- [x] `npm run format:check`
- [x] `npm run lint`
- [x] `npm run typecheck`
- [x] `npm run test -- components/dashboard/DashboardClient.test.tsx`
- [x] `npm run test`

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors.
- [x] My commits follow the Conventional Commits format.
- [x] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have starred the repo.
- [x] I have made sure that I have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse quality standard.

Author: JhaSourav07

components/dashboard/DashboardClient.test.tsx

@@ -1,6 +1,7 @@
 /* eslint-disable @typescript-eslint/no-explicit-any */
 import { render, screen, fireEvent, waitFor } from '@testing-library/react';
 import { describe, expect, it, vi, beforeEach } from 'vitest';
+import { toast } from 'sonner';
 import DashboardClient from './DashboardClient';
 
 vi.mock('next/navigation', () => ({
@@ -15,6 +16,14 @@ vi.mock('next/navigation', () => ({
   }),
 }));
 
+vi.mock('sonner', () => ({
+  toast: {
+    success: vi.fn(),
+    error: vi.fn(),
+    info: vi.fn(),
+  },
+}));
+
 // Mock framer-motion to avoid animation issues in tests
 vi.mock('framer-motion', () => ({
   motion: {
@@ -205,6 +214,7 @@ const mockPeriod = {
 describe('DashboardClient', () => {
   beforeEach(() => {
     vi.restoreAllMocks();
+    vi.clearAllMocks();
   });
 
   it('renders standard single profile view by default', () => {
@@ -327,6 +337,46 @@ describe('DashboardClient', () => {
     const generateLink = screen.getByRole('link', { name: /generate your own/i });
     expect(generateLink.getAttribute('href')).toBe('/');
   });
+
+  it('shows a success toast after the dashboard link is copied', async () => {
+    const writeText = vi.fn().mockResolvedValue(undefined);
+    Object.defineProperty(navigator, 'clipboard', {
+      value: { writeText },
+      configurable: true,
+    });
+
+    render(
+      <DashboardClient initialData={mockInitialData} username="Shivangi1515" period={mockPeriod} />
+    );
+
+    fireEvent.click(screen.getByRole('button', { name: /^share$/i }));
+
+    await waitFor(() => {
+      expect(writeText).toHaveBeenCalledWith(window.location.href);
+      expect(toast.success).toHaveBeenCalledWith('Link copied to clipboard!');
+    });
+  });
+
+  it('shows an error toast when the dashboard link copy fails', async () => {
+    const writeText = vi.fn().mockRejectedValue(new Error('Permission denied'));
+    Object.defineProperty(navigator, 'clipboard', {
+      value: { writeText },
+      configurable: true,
+    });
+
+    render(
+      <DashboardClient initialData={mockInitialData} username="Shivangi1515" period={mockPeriod} />
+    );
+
+    fireEvent.click(screen.getByRole('button', { name: /^share$/i }));
+
+    await waitFor(() => {
+      expect(writeText).toHaveBeenCalledWith(window.location.href);
+      expect(toast.error).toHaveBeenCalledWith('Failed to copy dashboard link');
+    });
+    expect(toast.success).not.toHaveBeenCalledWith('Link copied to clipboard!');
+  });
+
   // =========================================================================
   // ISSUE OBJECTIVE: Verify error is shown when comparing with same username
   // =========================================================================

components/dashboard/DashboardClient.tsx

@@ -477,6 +477,15 @@ export default function DashboardClient({
     }
   };
 
+  const handleShareDashboard = async () => {
+    try {
+      await navigator.clipboard.writeText(window.location.href);
+      toast.success('Link copied to clipboard!');
+    } catch {
+      toast.error('Failed to copy dashboard link');
+    }
+  };
+
   // ------------------------------------------------------------
   // Compare Mode Statistics Calculations
   // ------------------------------------------------------------
@@ -610,10 +619,7 @@ export default function DashboardClient({
 
           <RefreshButton username={username} />
           <button
-            onClick={() => {
-              navigator.clipboard.writeText(window.location.href);
-              toast.success('Link copied to clipboard!');
-            }}
+            onClick={handleShareDashboard}
             className="flex items-center gap-2 rounded-xl border border-black/10 px-4 py-2 text-sm font-semibold hover:bg-gray-100 dark:hover:bg-zinc-800 transition"
           >
             <Share2 size={16} />