Playlists: Fix 'invalid byte sequence' error when subscribing (#4887)

In Crystal, handling multi-byte sequences in UTF-8 requires understanding that
slicing by bytes can lead to invalid sequences if the slicing isn't aligned
with character boundaries. In this case, attempting to slice a string by bytes
can cut through multi-byte UTF-8 sequences, leading to invalid sequences.

To avoid this, strings should be sliced based on characters rather than bytes.

Fixes issue 4886

src/invidious/playlists.cr

@@ -270,7 +270,7 @@ end
 
 def subscribe_playlist(user, playlist)
   playlist = InvidiousPlaylist.new({
-    title:       playlist.title.byte_slice(0, 150),
+    title:       playlist.title[..150],
     id:          playlist.id,
     author:      user.email,
     description: "", # Max 5000 characters