Fix subdomain mode and content negotiation issues #19

Summary

Several issues were discovered when testing subdomain mode with solid.social:

1. Container content negotiation - Container listings always returned JSON-LD even when Accept: text/turtle was requested, causing mashlib parse errors
2. Subdomain URL generation - Pod creation and registration generated path-based URLs instead of subdomain-based URLs when subdomain mode was enabled
3. CORS DPoP header - The DPoP header was not in the allowed CORS headers list, breaking Solid-OIDC authentication
4. .ttl extension handling - Files with .ttl extension were not served as Turtle format

Reproduction

1. Enable subdomain mode in config
2. Create a new account
3. Visit the pod root (e.g., https://alice.example.com/)
4. Mashlib fails with parse errors because container listings return JSON-LD when Turtle is requested

Fix

• Add content negotiation support for container listings without index.html
• Pass correct podUri and issuer to createPodStructure in subdomain mode
• Add DPoP to Access-Control-Allow-Headers
• Check URL extension for .ttl files and serve Turtle regardless of Accept header

Files Changed

• src/handlers/resource.js - Container conneg and .ttl extension handling
• src/handlers/container.js - Pass podUri/issuer correctly
• src/idp/interactions.js - Build subdomain URLs during registration
• src/ldp/headers.js - Add DPoP to CORS headers