Add the Waitlist product template (product-waitlist) #257

Part of #5. For pre-launch posts that collect sign-ups.

Idea
"Join the waitlist" sits at the top as a small label in the accent color, with the product name large under it ("Notewell") and a one-line pitch under that ("Notes that link themselves"). Lower down, a big number shows how many people are waiting ("2,148") with a short label next to it ("people on the list"). An optional launch window ("Opening in November") and the site name sit at the bottom.

Template details

• File: src/templates/product/waitlist.ts, id product-waitlist
• Fields: commonFields.title (the product name), commonFields.description (the pitch), count (text, optional, default "2,148"), countLabel (text, optional, default "people on the list"), launchWindow (text, optional, default "Opening in November"), commonFields.siteName, commonFields.bgColor, commonFields.accentColor, commonFields.textColor

Where to look

• Closest reference: src/templates/product/announcement.ts (product name, pitch and launch date), src/templates/newsletter/subscriber-count.ts (a large count with a label)

Acceptance criteria

• Long product names and pitches shrink with autoFontSize() and then truncate, and an empty description, count, countLabel, launchWindow or site name (or only spaces) hides cleanly
• When count is empty its label hides too, and accent text stays readable on a light background
• Re-exported in src/templates/product/index.ts and registered in the Product block of src/templates/registry.ts
• Meets the shared template checklist in Add new card-style OG image templates #5

One template per contributor at a time, so more people get a chance.

Want to work on this? Comment below and I'll assign it to you. Please take one issue at a time, branch off dev, and open your PR against dev with Closes #257 in the description.