fix(signin): prevent white flash before dark background loads

[mallya-m]

What does this PR do?

Fixes the white flash visible on the sign-in page before React hydrates. Before this fix, the browser briefly renders a white background before the dark #080808 background kicks in.

Related issue

Closes #1040

Changes made

• Added src/app/auth/layout.tsx scoped to the /auth route
• Injects a style tag setting html/body background to #080808 before paint
• Adds color-scheme: dark to fix scrollbar and system UI colors
• Also fixed pre-existing empty src/app/api/public/privacy/route.ts which was causing build failures across all PRs

How to test

1. Open /auth/signin in a fresh browser tab
2. No white flash before the dark background appears
3. Hard refresh (Ctrl+Shift+R) — still no flash

Screenshots

N/A — visual timing fix

[vercel]

@mallya-m is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[mallya-m]

@Priyanshu-byte-coder could you please add gssoc:approved, level:intermediate, and type:bug labels? Thank you!

[github-actions]

🎉 Merged! Thanks for contributing to DevTrack.

If the project has been useful to you, a ⭐ star on the repo is the easiest way to support it — it helps DevTrack get discovered by more developers.

Keep an eye on open issues for your next contribution!