Add admin credentials to interactive site creation flow

[bcotrim]

Related issues

• Related to Add support for Blueprints login #2527

Proposed Changes

• Add interactive prompts for admin username, password, and email during studio site create
• Username prompt shows "admin" as default and validates input
• Password prompt is masked with asterisks, allows empty input for auto-generation
• Email prompt shows "admin@localhost.com" as default
• Credentials still work via CLI flags when non-interactive mode is used
• All prompts respect precedence: CLI flags override interactive input

Testing Instructions

• Run studio site create in interactive mode and verify:
  • You're prompted for admin username (after HTTPS setup)
  • You're prompted for admin password (masked input, press Enter to auto-generate)
  • You're prompted for admin email with default admin@localhost.com
  • Pressing Enter uses the default values
  • Invalid usernames show validation errors
  • Invalid emails show validation errors
• Run studio site create --admin-username custom --admin-password secret and verify:
  • Interactive prompts are skipped (flags are respected)
  • Site is created with provided credentials
• Test with Blueprints containing login credentials and verify they still work

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing 7ea98b5 vs trunk

site-editor

Metric	trunk	7ea98b5	Diff	Change
load	1426.00 ms	1444.00 ms	+18.00 ms	⚪ 0.0%

site-startup

Metric	trunk	7ea98b5	Diff	Change
siteCreation	7080.00 ms	7119.00 ms	+39.00 ms	⚪ 0.0%
siteStartup	3945.00 ms	3941.00 ms	-4.00 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)