ads: add a persistent "Monetize a new site" button on /ads/slots

[ralyodio]

Problem

On /ads/slots ("Monetize your site") there was no way to start monetizing another site once you already had projects. The only link to project creation lived inside the empty state (projects.length === 0), which never renders when you have projects — so with sites already added, the page showed the list with no "add a site" affordance. (Not a new regression — the link was always empty-state-only — but it reads as a missing button.)

Fix

• Persistent header button — "Monetize a new site" in the Monetize header, always visible.
• Closes the loop — /projects/new now accepts ?next=<relative-path> and returns there after creating (guarded to same-site relative paths — no open redirect). So the button sends you to /projects/new?next=/ads/slots, and after creating you land back on /ads/slots with the new site ready to Enable ads.
• Empty-state "Create one" link is now return-aware too.

Changes

• app/(app)/ads/slots/page.tsx — header button + return-aware empty-state link
• app/(app)/projects/new/page.tsx — read next from searchParams (Next 16 async), pass to form
• app/(app)/projects/new/form.tsx — redirect to next when it's a safe relative path

Verified

• tsc --noEmit clean
• next build compiles; /ads/slots and /projects/new build as expected

🤖 Generated with Claude Code

[github-actions]

vu1nz Security Review

0 finding(s) in PR #?

No security issues found.