Add the Deprecation Notice SaaS template (saas-deprecation) #277

Part of #5. For announcing that an API, a feature or a version is going away.

Idea
A small "Deprecated" label in the accent color at the top, then what's going away, large and struck through ("API v1"), with an arrow to what replaces it ("API v2"). Under that, the sunset date ("Shuts down on March 31, 2027") and an optional one-line note ("The migration guide is in the docs"), with the site name at the bottom.

Template details

• File: src/templates/saas/deprecation.ts, id saas-deprecation
• Fields: commonFields.title (what's going away), replacement (text, optional, default "API v2"), sunsetDate (text, optional, default "Shuts down on March 31, 2027"), commonFields.description (the note), commonFields.siteName, commonFields.bgColor, commonFields.accentColor, commonFields.textColor

Where to look

• Closest reference: src/templates/saas/maintenance.ts (a dated notice), src/templates/event/rescheduled.ts (an old value struck through, with the new one next to it)

Acceptance criteria

• Long names shrink with autoFontSize() and then truncate, and an empty replacement, sunsetDate, description or site name (or only spaces) hides cleanly, with the arrow hidden when there's no replacement
• The label and any accent text stay readable on a light background
• Re-exported in src/templates/saas/index.ts and registered in the SaaS 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 #277 in the description.