fix: carddav proppatch support, sync-collection, validate vcard, if-n…

[shaunwarman]

…one-match and incomplete vcard

Fix Critical CardDAV Bugs and Add RFC Compliance

Fixes

1. PROPPATCH Support - Implemented missing PROPPATCH method for updating address book properties (displayname, description, color)
2. vCard Validation - Added RFC 6350 compliance checks: validates VERSION, FN, BEGIN/END markers, and 1MB size limit
3. If-None-Match Header - Added support for If-None-Match: * to prevent accidental overwrites (RFC 2616)
4. Sync-Collection - Fixed incremental sync to return only modified contacts instead of all contacts using updated_at timestamps
5. URL Consistency - Fixed username/email inconsistency in address book URL generation

Changes

• routes/carddav/index.js - PROPPATCH handler, If-None-Match check, sync-collection timestamp filtering
• helpers/carddav-xml.js - New validateVCard() function with RFC 6350 validation
• helpers/ensure-default-address-book.js - Fixed URL to use username instead of email

Tests Added

• 7 new tests covering vCard validation, If-None-Match, PROPPATCH, and sync-collection

All changes maintain backward compatibility and improve RFC 6352/6350 compliance.