Expose the grid columns picker in SubscriptionsView and ManageChannelsView
inline sheets, and track viewWidth in ChannelView's tvOS GeometryReaders so
the shared ViewOptionsSheet can compute a meaningful column range.
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.
On tvOS, ContentUnavailableView inside a Group doesn't expand to fill
available space — it sizes to content and aligns top-leading. Add
.frame(maxWidth: .infinity, maxHeight: .infinity) to all instances
so they center correctly in their parent containers.
Remove 32 non-dotted keys (16 unused format specifiers, 16 word keys)
and replace with properly namespaced dotted keys following the existing
convention (common.*, player.*, search.*).
