feat(app): cite sources in APA, MLA or Chicago, and give studysets a bibliography

A source is now shared by learning paths and studysets (core/citation/):
authors as full names, year, title, what it is a part of, publisher,
ISBN, link, and a citation kept as a file brought it. formatCitation
cites it in APA 7, MLA 9 or Chicago 18: it inverts and abbreviates
names itself (particles, suffixes, names written as already cited stay),
sets a work of its own in italics and a part of something apart from
the whole, and writes a missing year as "n.d." in APA and Chicago.

- Settings: a Citations card per device, with a preview and a link to
  the style's own guide.
- Studysets: a Bibliography button opens the works cited, to read and,
  behind its pencil, to change (studyset schema 4).
- Steps: sources edited through the same sheet, opened from an
  "N sources" button in the runner (step schema 2; schema 1 reads on).
- Path import: steps' sources, text kept as written, the old format's
  "citation" read as what a source is part of.

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

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -57,6 +57,8 @@
   "studysetStar": "Star",
   "studysetUnstar": "Unstar",
   "studysetOptions": "Options",
+  "studysetBibliography": "Bibliography",
+  "studysetBibliographyIntro": "Works cited across this studyset, in the citation style chosen in Settings.",
   "studysetMenuEdit": "Edit",
   "studysetMenuArchive": "Archive",
   "studysetMenuUnarchive": "Unarchive",
@@ -140,15 +142,41 @@
   "stepFeedbackRight": "Feedback after a right answer",
   "stepFeedbackWrong": "Feedback after a wrong answer",
   "stepFeedbackNone": "None",
-  "stepSources": "Sources",
-  "stepAddSource": "Add source",
-  "stepRemoveSource": "Remove source",
-  "stepSourceAuthors": "Authors",
-  "stepSourceYear": "Year",
-  "stepSourceTitle": "Title",
-  "stepSourceUrl": "Link",
-  "stepSourcesCount": "{count, plural, =1{1 source} other{{count} sources}}",
-  "@stepSourcesCount": {"placeholders": {"count": {"type": "int"}}},
+  "sources": "Sources",
+  "sourcesCount": "{count, plural, =1{1 source} other{{count} sources}}",
+  "@sourcesCount": {"placeholders": {"count": {"type": "int"}}},
+  "sourcesNone": "No sources cited yet.",
+  "sourcesEdit": "Edit sources",
+  "sourcesEditDone": "Done editing",
+  "sourceAdd": "Add a source",
+  "sourceEdit": "Edit source",
+  "sourceRemove": "Remove source",
+  "sourceRemoved": "Source removed",
+  "sourceSheetAdd": "Add",
+  "sourceAuthors": "Authors",
+  "sourceAuthorsHint": "Full names, one per line, e.g. Mark A. McDaniel",
+  "sourceYear": "Year",
+  "sourceYearHint": "e.g. 2014",
+  "sourceTitle": "Title",
+  "sourceTitleHint": "Title of the work",
+  "sourceContainer": "Published in",
+  "sourceContainerHint": "Journal, book or website it is part of, with pages",
+  "sourceContainerHelper": "Leave empty for a work of its own, such as a book.",
+  "sourcePublisher": "Publisher",
+  "sourcePublisherHint": "e.g. Belknap Press",
+  "sourceIsbn": "ISBN",
+  "sourceIsbnHint": "e.g. 978-0-674-72901-8",
+  "sourceUrl": "Link",
+  "sourceUrlHint": "https://…",
+  "sourceImportedAs": "Imported as: {citation}",
+  "@sourceImportedAs": {"placeholders": {"citation": {"type": "String"}}},
+  "sourceIsbnLine": "ISBN: {isbn}",
+  "@sourceIsbnLine": {"placeholders": {"isbn": {"type": "String"}}},
+  "sourceAvailableAt": "Available at:",
+  "sourceEmpty": "Empty reference",
+  "sourceDiscardTitle": "Discard changes?",
+  "sourceDiscardMessage": "What you typed for this source will be lost.",
+  "sourceDiscardConfirm": "Discard",
   "stepConflict": "This step was changed differently on another device. Keep one version to go on writing.",
   "stepFieldTitle": "Title",
   "stepFieldBody": "Text",
@@ -276,6 +304,14 @@
   "settingsAppearanceDark": "Dark",
   "settingsLanguage": "Language",
   "settingsLanguageDevice": "Device language",
+  "settingsCitations": "Citations",
+  "settingsCitationsDescription": "How sources and studysets' bibliographies are cited on this device. Type authors' full names: each style abbreviates and orders them itself.",
+  "settingsCitationStyle": "Style",
+  "settingsCitationStyleApa": "APA (7th edition)",
+  "settingsCitationStyleMla": "MLA (9th edition)",
+  "settingsCitationStyleChicago": "Chicago (18th edition)",
+  "settingsCitationPreview": "Preview",
+  "settingsCitationGuide": "Official guide",
   "settingsVersion": "Version {version} (build {build})",
   "@settingsVersion": {"placeholders": {"version": {"type": "String"}, "build": {"type": "String"}}},
   "syncTitle": "Sync",
@@ -1034,6 +1070,7 @@
   "@pathImportTooFew": {"placeholders": {"detail": {"type": "String"}}},
   "pathImportMalformedNumber": "a malformed number.",
   "pathImportTitleNotText": "a title must be text.",
+  "pathImportMalformedSource": "a source must be an object or text, and its parts text.",
   "pomodoroTopBarHint": "The timer can count down in GNOME's top bar, with its buttons, once the AppIndicator extension is on.",
   "pomodoroTopBarSetUp": "Set up",
   "pomodoroTopBarDismiss": "Don't suggest again"