feat(jellyfin): the instance suggests a Jellyfin server, signed in to with a tap

An instance now names a Jellyfin server for its people to learn from
(instance.info().jellyfinUrl): its operator's JELLYFIN_URL (`none`: none),
else https://jellyfin. on the domain registered for its public host, as
learn.halkyon.eu suggests jellyfin.halkyon.eu, once that answers as a
Jellyfin server. The registered domain comes from Mozilla's Public Suffix
List, kept in the server's data folder, so an instance at example.co.uk
never names someone else's jellyfin.co.uk. The instance looks as it
starts and again hourly when asked; the app never reaches for a guessed
address.

In the app, Add material offers From Jellyfin as soon as the instance
suggests a server, before anyone signed in to one. The sign-in fills the
server in and finds it by itself, and lists the people the server shows
on its login screen: one without a password, as a demo's, signs in with
a tap; one with a password is named in the form, which asks for it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

deploy/.env.example

@@ -29,6 +29,12 @@ GITHUB_CLIENT_SECRET=
 # Running modified code, set it to where your modified source is published.
 SOURCE_URL=
 
+# The Jellyfin server the app suggests to this instance's people, who watch and listen
+# to learn from it (Halkyon Learn's Watch screen). Empty: `https://jellyfin.` on the
+# domain registered for this instance (learn.example.org → jellyfin.example.org), if a
+# Jellyfin server answers there; `none`: no suggestion.
+JELLYFIN_URL=
+
 # How much room people's images and sounds may take on this instance, in MiB. Empty: the
 # defaults, 100 for an anonymous person (anonymous accounts are meant to become identified
 # ones), 500 for one signed in with GitHub, and 15000 for everyone together (0: no limit