fix: viper issue in parsing sync headers

[raflFaisal]

Summary

Viper cannot parse StringToString flags from environment variables. This adds a manual fallback parser for FLAGD_SYNC_HEADERS and canonicalizes header keys for case-insensitive merging.

Changes

• flagd/cmd/start.go — Add parseSyncHeaders() fallback that manually parses key=value,key=value format; use case-insensitive comparison when merging global headers
• flagd/cmd/start_test.go — Unit tests for the header parser
• core/pkg/sync/http/http_sync.go — Canonicalize header keys with http.CanonicalHeaderKey
• core/pkg/sync/http/http_sync_test.go — Consolidate header tests into table-driven format