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_en.arb

@@ -240,6 +240,19 @@
   "sourceDiscardTitle": "Discard changes?",
   "sourceDiscardMessage": "What you typed for this source will be lost.",
   "sourceDiscardConfirm": "Discard",
+  "sourceKind": "Kind of source",
+  "sourceKindWork": "Book or article",
+  "sourceKindWebsite": "Website",
+  "sourceKindVideo": "YouTube video",
+  "sourceWebsiteName": "Website",
+  "sourceWebsiteNameHint": "Its name, e.g. Wikipedia",
+  "sourcePageTitleHint": "Title of the page",
+  "sourceChannel": "Channel",
+  "sourceChannelHint": "Who published it on YouTube",
+  "sourceVideoTitleHint": "Title of the video",
+  "sourceVideoLink": "Link to the video",
+  "sourceVideoLinkInvalid": "Not a link to a YouTube video",
+  "sourcePlay": "Play video",
   "stepConflict": "This step was changed differently on another device. Keep one version to go on writing.",
   "stepFieldTitle": "Title",
   "stepFieldBody": "Text",