Delete declarations that Periphery reports unused on every platform that
compiles them (iOS, macOS, tvOS), including nine views used only by their
own previews. Platform-gated code still used on another platform, device-
only MPV render paths, migration models, CloudKit record names, App
Intents metadata and unused parameters are kept.
yattee-server returns direct YouTube CDN URLs in the storyboard `url`
and `templateUrl` fields instead of an Invidious-style VTT proxy path.
Two resulting issues:
- `Storyboard.directSheetURL` was replacing the whole `M$M` token with
just the index, producing `.../0.jpg` (404) instead of `.../M0.jpg`.
Replace `M$M` with `M\(index)` to preserve the literal `M` prefix;
matching the full token also avoids clobbering `$M` sequences that
may appear in `sigh=rs$...` query params.
- The download code fetched `proxyUrl` as if it were a WebVTT file;
with yattee-server that downloads a JPEG that fails UTF-8 parsing.
Skip the VTT round-trip when `proxyUrl` obviously points at an image.
Also align the on-disk filename with the local-playback template
(`sb_M$M.jpg` → `sb_M{N}.jpg`) so offline seek-bar previews resolve,
and add [Storyboard] debug logs at each decision point so future
failures can be diagnosed without guessing.
