feat: add theme presets with graceful fallback for user customization

[sureshsuriya]

Description

Fixes #7251

This PR adds multiple predefined theme presets with one-click switching, making it easier for users to customize their badges without manually configuring colors.

Changes

• Added two new theme presets: mono and galaxy.

• Exported a shared THEME_PRESETS list for frontend usage.

• Updated theme handling to gracefully fall back to the default theme when an unknown theme is provided.

• Added an X-Theme-Warning response header instead of returning a 400 Bad Request for invalid theme names.

• Added a Theme Presets gallery on the landing page with one-click theme selection.

• Updated the generated embed URL to include the selected theme parameter.

• Added/updated tests covering:

  • Theme preset availability.
  • Unknown theme fallback.
  • Warning header behavior.

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.
• 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.
• (Recommended) I joined the CommitPulse Discord community for contributor discussions, mentorship, and faster PR support.

[vercel]

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[retenta-bot]

This pull request enhances user experience by allowing easy theme customization through predefined presets and implementing a fallback for unknown themes. This aligns with our previous decision to implement visually themed fallback SVGs for API errors, ensuring consistency in user experience across the platform. Thank you for your contribution, and I look forward to your updates!

[github-actions]

📦 Next.js Bundle Size Report (Gzipped Sizes)

Asset	PR Size	Base Size	Difference	Status
static/chunks/app/page-[hash].js	21.25 KB	21.04 KB	+0.21 KB (+1.00%)	🔴 Regression
static/css/[hash].css	41.11 KB	41.08 KB	+0.02 KB (+0.05%)	🔴 Regression

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	3985.78 KB	3984.83 KB	+0.95 KB (+0.02%)
Total CSS	331.61 KB	331.41 KB	+0.20 KB (+0.06%)

[Aamod007]

Thanks for adding the theme presets! The graceful fallback mechanism and the new presets are solid additions to the customization UI.

However, I noticed that a file named \diff.txt\ was accidentally committed to this pull request. Please remove this file from your branch and update the PR.

Looking forward to merging this once that file is removed!

Label Justification:

• \level:beginner: Adding theme presets and updating validation falls into beginner frontend/backend tasks.
• \quality:clean: Applied as default pending removal of the rogue file.
• \ ype:feature: Adds new preset options to the generator UI.
• \mentor:Aamod007: Assigned as required.