feat: add synthwave theme

[Priyanshu-byte-coder]

Description

Fixes #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

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

Visual Preview

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:

• 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 starred 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).

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Project	Deployment	Actions	Updated (UTC)
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[JhaSourav07]

@Priyanshu-byte-coder

PR description is not matching.

## 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).

this is the PR description also the PNG which you have added is not right, i would request you to update it as the screenshot of SVG generated by the website for you theme.

[Priyanshu-byte-coder]

@JhaSourav07

[Priyanshu-byte-coder]

@JhaSourav07 addressed both comments:

• Removed synthwave-preview.png — only synthwave.png remains now
• THEMES.md already has ![Synthwave](assets/themes/synthwave.png) — the image now shows the actual SVG badge output (real screenshot from localhost)
• Updated PR description visual preview to use synthwave.png directly

Ready for re-review!

[JhaSourav07]

@Priyanshu-byte-coder

Thanks for the contribution.
I am merging it now.

[JhaSourav07]

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