fix: prevent 'admin with no permissions' on fresh install

[baeyc0510]

Fixes permission issues when user-policy.json creation fails or is missing.

Changes:

1. init.go: Make createDefaultPolicy() failure fatal

   • Previously: warning only, continued execution
   • Now: exits with detailed error message
   • Provides actionable guidance (antivirus, permissions, disk space)

2. manager.go: Add safe RBAC fallback in LoadPolicy()

   • Previously: returned RBAC=nil when file missing
   • Now: returns minimal admin role with full permissions
   • Prevents "admin with no permissions" scenario in dashboard

3. manager.go: Enhanced error messages for SavePolicy()

   • Added specific causes for write failures
   • Windows-specific guidance (antivirus, OneDrive, UAC)

Root Cause:

• On Windows, file write can fail due to:
  • Antivirus real-time protection scanning binary
  • OneDrive/Dropbox sync conflicts
  • Controlled Folder Access (ransomware protection)
  • Transient I/O errors
• Silent failure left .env with CURRENT_ROLE=admin but no policy file
• Dashboard loaded RBAC=nil → permission check failed

Impact:

• Prevents confusing UX where admin role has no edit buttons
• Provides clear error messages to help users resolve issues
• Fails fast instead of creating inconsistent state