feat(citation): sources are books and articles, websites or YouTube videos

A source was a book's or an article's parts, however a website or a
video had to be squeezed into them. Adding one to a bibliography or a
step now asks what it is, and its form asks for what cites it: a book
or an article its parts as before; a website the page's link, title,
the site's name, authors and year; a YouTube video its link (checked to
be one), title, channel and year. Websites and videos need their link,
marked as required. A video is cited as APA cites one ("[Video]", a
part of YouTube) and plays in the app where it can, else on YouTube.

The kind is kept in the source's JSON, left out for a book or an
article; a kind this app doesn't know reads as a work. The studyset's
payload goes to schema 6 and a step's to schema 4 for it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -206,6 +206,19 @@
   "sourceDiscardTitle": "Änderungen verwerfen?",
   "sourceDiscardMessage": "Was du für diese Quelle eingegeben hast, geht verloren.",
   "sourceDiscardConfirm": "Verwerfen",
+  "sourceKind": "Art der Quelle",
+  "sourceKindWork": "Buch oder Artikel",
+  "sourceKindWebsite": "Website",
+  "sourceKindVideo": "YouTube-Video",
+  "sourceWebsiteName": "Website",
+  "sourceWebsiteNameHint": "Ihr Name, z. B. Wikipedia",
+  "sourcePageTitleHint": "Titel der Seite",
+  "sourceChannel": "Kanal",
+  "sourceChannelHint": "Wer es auf YouTube veröffentlicht hat",
+  "sourceVideoTitleHint": "Titel des Videos",
+  "sourceVideoLink": "Link zum Video",
+  "sourceVideoLinkInvalid": "Kein Link zu einem YouTube-Video",
+  "sourcePlay": "Video abspielen",
   "stepConflict": "Dieser Schritt wurde auf einem anderen Gerät anders geändert. Behalte eine Version, um weiterzuschreiben.",
   "stepFieldTitle": "Titel",
   "stepFieldBody": "Text",