fix(templates): handle optional PKCE in retry links

The "Request a new code" and "Try Again" links were outputting
code_challenge=None when PKCE was not used, causing the retry
to fail with validation errors.

Now only includes PKCE parameters if code_challenge has a value.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

uv.lock

@@ -431,7 +431,7 @@ wheels = [
 
 [[package]]
 name = "gondulf"
-version = "1.0.0rc1"
+version = "1.0.1"
 source = { editable = "." }
 dependencies = [
     { name = "aiosmtplib" },