Replace the YatteeServerAPI setAuthHeader/buildHeaders pattern (which was
race-prone on the shared actor across multiple instances) with a generic
mechanism: HTTPClient now supports a defaultHeaders dictionary applied to
every request, and ContentService builds a per-instance HTTPClient with the
basic-auth Authorization header baked in whenever credentials are configured.
The same code path now works uniformly for Invidious, Piped, PeerTube, and
Yattee Server, so any instance sitting behind a reverse proxy that requires
HTTP Basic Auth can be authenticated regardless of backend type. Cached
default API actors are still reused when no basic-auth header is needed.
Renames YatteeServerCredentialsManager → BasicAuthCredentialsManager so the
same Keychain-backed username/password storage can be reused for any instance
type that sits behind a reverse proxy requiring HTTP Basic Auth. Adds a
one-time migration that moves existing items from the legacy
'com.yattee.yatteeserver' Keychain service to 'com.yattee.basicauth',
preserving the iCloud-sync attribute. No behavior change for end users.
Downloads were using direct YouTube CDN URLs even when proxiesVideos
was enabled. Apply the same proxyStreamsIfNeeded used by the player
to the download code path in ContentService.
