feat: add synthwave theme (JhaSourav07#83)

## Description

Fixes JhaSourav07#56

Adds a `synthwave` theme — retro 80s-inspired palette with deep indigo
background (`#0d0221`), neon pink accent glow (`#ff2d78`), and crisp
off-white text (`#f8f8f2`).

## Pillar

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

## Visual Preview

![Synthwave Theme
Preview](https://github.com/Priyanshu-byte-coder/commitpulse/blob/feat/synthwave-theme/assets/themes/synthwave.png?raw=true)

**Colors:**

| Key | Hex | Role |
|---|---|---|
| `bg` | `#0d0221` | Deep space indigo |
| `text` | `#f8f8f2` | Off-white |
| `accent` | `#ff2d78` | Neon pink glow |

**Usage:** `/api/streak?user=yourusername&theme=synthwave`

## 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 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 "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).

Author: JhaSourav07

THEMES.md

@@ -81,3 +81,11 @@ Usage: `/api/streak?user=yourusername&theme=rose`
 ![Nord](assets/themes/nord.png)
 
 Usage: `/api/streak?user=yourusername&theme=nord`
+
+## 🌆 Synthwave
+
+![Synthwave](assets/themes/synthwave.png)
+
+Retro 80s-inspired palette — deep indigo background, neon pink glow, crisp white text.
+
+Usage: `/api/streak?user=yourusername&theme=synthwave`

assets/themes/synthwave.png

@@ -52,4 +52,9 @@ export const themes: Record<string, BadgeTheme> = {
     text: 'd8dee9',
     accent: '88c0d0',
   },
+  synthwave: {
+    bg: '0d0221',
+    text: 'f8f8f2',
+    accent: 'ff2d78',
+  },
 };