feat(themes): add gruvbox preset with dark palette (JhaSourav07#170)

## Description

Fixes JhaSourav07#148

## Pillar



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

## Visual Preview
<img width="1152" height="648" alt="gruvbox"
src="https://github.com/user-attachments/assets/56578bfd-7533-4a17-afc6-f539fe6af23d"
/>

## 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 (CI will fail otherwise).
- [x ] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [x ] I have updated `README.md` if I added a new theme or URL
parameter.
- [x ] I have started 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 "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.

Author: JhaSourav07

README.md

@@ -106,6 +106,7 @@ URL Parameter > Theme Default > System Fallback
 | `dracula`          | Dracula Pro              | `282a36` | `bd93f9` | `f8f8f2` |
 | `github`           | GitHub green             | `0d1117` | `238636` | `ffffff` |
 | `light`            | Clean & minimal          | `ffffff` | `0969da` | `24292f` |
+| `gruvbox`          | retro warm dark          | `282828` | `fe8019` | `ebdbb2` |
 | `random`           | Surprise theme on reload | _varies_ | _varies_ | _varies_ |
 
 > **`auto` uses CSS `@media (prefers-color-scheme)`** inside the SVG so the badge switches between the `light` and `dark` palettes based on the viewer's OS setting — no JavaScript required. This is ideal for GitHub profile READMEs where visitors may use either mode.

THEMES.md

@@ -44,6 +44,14 @@ Usage: `/api/streak?user=yourusername&theme=github`
 
 ---
 
+## 🟠 Gruvbox
+
+![Gruvbox](assets/themes/gruvbox.png)
+
+Usage: `/api/streak?user=yourusername&theme=gruvbox`
+
+---
+
 ## 🌊 Ocean
 
 ![Ocean](assets/themes/ocean.png)

assets/themes/gruvbox.png

@@ -57,6 +57,11 @@ export const themes: Record<string, BadgeTheme> = {
     text: 'f8f8f2',
     accent: 'ff2d78',
   },
+  gruvbox: {
+    bg: '282828',
+    text: 'ebdbb2',
+    accent: 'fe8019',
+  },
 };
 
 // Auto-theme pairs: the SVG switches between these two palettes