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_localizations_en.dart

@@ -193,6 +193,13 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get studysetOptions => 'Options';
 
+  @override
+  String get studysetBibliography => 'Bibliography';
+
+  @override
+  String get studysetBibliographyIntro =>
+      'Works cited across this studyset, in the citation style chosen in Settings.';
+
   @override
   String get studysetMenuEdit => 'Edit';
 
@@ -454,28 +461,10 @@ class AppLocalizationsEn extends AppLocalizations {
   String get stepFeedbackNone => 'None';
 
   @override
-  String get stepSources => 'Sources';
+  String get sources => 'Sources';
 
   @override
-  String get stepAddSource => 'Add source';
-
-  @override
-  String get stepRemoveSource => 'Remove source';
-
-  @override
-  String get stepSourceAuthors => 'Authors';
-
-  @override
-  String get stepSourceYear => 'Year';
-
-  @override
-  String get stepSourceTitle => 'Title';
-
-  @override
-  String get stepSourceUrl => 'Link';
-
-  @override
-  String stepSourcesCount(int count) {
+  String sourcesCount(int count) {
     String _temp0 = intl.Intl.pluralLogic(
       count,
       locale: localeName,
@@ -485,6 +474,104 @@ class AppLocalizationsEn extends AppLocalizations {
     return '$_temp0';
   }
 
+  @override
+  String get sourcesNone => 'No sources cited yet.';
+
+  @override
+  String get sourcesEdit => 'Edit sources';
+
+  @override
+  String get sourcesEditDone => 'Done editing';
+
+  @override
+  String get sourceAdd => 'Add a source';
+
+  @override
+  String get sourceEdit => 'Edit source';
+
+  @override
+  String get sourceRemove => 'Remove source';
+
+  @override
+  String get sourceRemoved => 'Source removed';
+
+  @override
+  String get sourceSheetAdd => 'Add';
+
+  @override
+  String get sourceAuthors => 'Authors';
+
+  @override
+  String get sourceAuthorsHint =>
+      'Full names, one per line, e.g. Mark A. McDaniel';
+
+  @override
+  String get sourceYear => 'Year';
+
+  @override
+  String get sourceYearHint => 'e.g. 2014';
+
+  @override
+  String get sourceTitle => 'Title';
+
+  @override
+  String get sourceTitleHint => 'Title of the work';
+
+  @override
+  String get sourceContainer => 'Published in';
+
+  @override
+  String get sourceContainerHint =>
+      'Journal, book or website it is part of, with pages';
+
+  @override
+  String get sourceContainerHelper =>
+      'Leave empty for a work of its own, such as a book.';
+
+  @override
+  String get sourcePublisher => 'Publisher';
+
+  @override
+  String get sourcePublisherHint => 'e.g. Belknap Press';
+
+  @override
+  String get sourceIsbn => 'ISBN';
+
+  @override
+  String get sourceIsbnHint => 'e.g. 978-0-674-72901-8';
+
+  @override
+  String get sourceUrl => 'Link';
+
+  @override
+  String get sourceUrlHint => 'https://…';
+
+  @override
+  String sourceImportedAs(String citation) {
+    return 'Imported as: $citation';
+  }
+
+  @override
+  String sourceIsbnLine(String isbn) {
+    return 'ISBN: $isbn';
+  }
+
+  @override
+  String get sourceAvailableAt => 'Available at:';
+
+  @override
+  String get sourceEmpty => 'Empty reference';
+
+  @override
+  String get sourceDiscardTitle => 'Discard changes?';
+
+  @override
+  String get sourceDiscardMessage =>
+      'What you typed for this source will be lost.';
+
+  @override
+  String get sourceDiscardConfirm => 'Discard';
+
   @override
   String get stepConflict =>
       'This step was changed differently on another device. Keep one version to go on writing.';
@@ -890,6 +977,31 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get settingsLanguageDevice => 'Device language';
 
+  @override
+  String get settingsCitations => 'Citations';
+
+  @override
+  String get settingsCitationsDescription =>
+      'How sources and studysets\' bibliographies are cited on this device. Type authors\' full names: each style abbreviates and orders them itself.';
+
+  @override
+  String get settingsCitationStyle => 'Style';
+
+  @override
+  String get settingsCitationStyleApa => 'APA (7th edition)';
+
+  @override
+  String get settingsCitationStyleMla => 'MLA (9th edition)';
+
+  @override
+  String get settingsCitationStyleChicago => 'Chicago (18th edition)';
+
+  @override
+  String get settingsCitationPreview => 'Preview';
+
+  @override
+  String get settingsCitationGuide => 'Official guide';
+
   @override
   String settingsVersion(String version, String build) {
     return 'Version $version (build $build)';
@@ -3091,6 +3203,10 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get pathImportTitleNotText => 'a title must be text.';
 
+  @override
+  String get pathImportMalformedSource =>
+      'a source must be an object or text, and its parts text.';
+
   @override
   String get pomodoroTopBarHint =>
       'The timer can count down in GNOME\'s top bar, with its buttons, once the AppIndicator extension is on.';