feat(customize): add dark/light preview background toggle for contribution graph

[armanpanigrahi59]

Add a Sun/Moon toggle button to the Live Preview section of the contribution graph embed preview page (/customize). Clicking the button switches the preview container background between GitHub dark (#0d1117) and GitHub light (#ffffff), letting users instantly visualise how their isometric SVG embed will appear in both dark and light README contexts.

Key design decisions:

• Toggle state (previewBg) is intentionally local (useState) and is never synced to URL params, so it has zero effect on the export snippet or the API request.
• The preview container background is driven by Tailwind utility classes chosen to match the exact GitHub README background colours.
• Sun icon shows when the page is in dark-preview mode (click → switch to light); Moon icon shows in light-preview mode (click → switch to dark).
• Button carries a descriptive �ria-label and itle that update together with the icon and label text.
• All i18n keys are added to all 9 locale files (en, de, es, fr, hi, ja, ko, pt, zh); locale parity tests pass.

Tests added (page.preview-bg-toggle.test.tsx, 7 cases):

1. Toggle button renders on page load.
2. Preview container initialises with data-preview-bg=dark.
3. First click switches container to data-preview-bg=light.
4. After switching to light, button shows Dark label.
5. Second click toggles back to data-preview-bg=dark.
6. Export snippet URL is not polluted by the toggle state.
7. history.replaceState is not called when toggling.

Fixes: #2425

Description

Fixes # (issue number)

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Bug fix, refactoring, docs)

Visual Preview

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• I have updated README.md if I added a new theme or URL parameter.
• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard (no raw elements, smooth animations, correct fonts).
• (Recommended) I joined the CommitPulse Discord community for contributor discussions, mentorship, and faster PR support.

[vercel]

@armanpanigrahi59 is attempting to deploy a commit to the jhasourav07's projects Team on Vercel.

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[retenta-bot]

This PR adds a valuable feature that enhances the user interface by allowing users to preview their contributions in both dark and light modes. It aligns well with our previous decision to improve the customization options in the studio, similar to the 'Copy SVG' button feature. Great job on ensuring comprehensive test coverage as well! Looking forward to your contributions. 😊

[github-actions]

📦 Next.js Bundle Size Report (Gzipped Sizes)

Asset	PR Size	Base Size	Difference	Status
static/chunks/6247-[hash].js	36.23 KB	35.85 KB	+0.37 KB (+1.04%)	🔴 Regression
static/chunks/app/customize/page-[hash].js	22.26 KB	21.80 KB	+0.47 KB (+2.14%)	🔴 Regression

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	4056.51 KB	4052.50 KB	+4.01 KB (+0.10%)
Total CSS	338.94 KB	338.94 KB	0 B

[Aamod007]

Dark/light preview background toggle for the contribution graph customizer — spans 11 files including locale updates. CI failing, please fix.

Labels: level:intermediate (11 files but mostly locale strings), quality:clean, type:feature, type:testing, mentor:Aamod007

[Aamod007]

Re-reviewing after noting the only CI failure is Vercel authorization (not code-related). All other checks (Format · Lint · Typecheck · Test, Production Build, CodeQL) pass. Approving — the Vercel deploy auth is a repo-level config issue, not a code problem.