feat(learn): Markdown files beside the PDFs, read formatted

A studyset's files take Markdown (.md, .markdown) as well as PDFs, in
the same synced rows with a kind (payload schema 3, written only for
Markdown, so older apps skip those rows). Upload a PDF becomes Upload a
file; a Markdown file must be UTF-8 text.

The reader opens one formatted and read-only through the editor's
Markdown reader, keeping fenced code, which that reader drops; the file
itself never changes, and its source text is a tap away, selectable too.
It has neither search, zoom nor screenshots. It reads in 100 parts by
its top-level blocks, so reading progress, deadlines and the cards and
notes tied to a place work as a PDF's pages do, shown as a share; its
outermost headings are its chapters.

Whats-New: Upload Markdown files beside your PDFs: they open formatted, with reading progress, deadlines and flashcards from what you select.
Whats-New-De: Lade Markdown-Dateien neben deine PDFs hoch: Sie öffnen sich formatiert, mit Lesefortschritt, Fristen und Karteikarten aus deiner Auswahl.

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

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations.dart

@@ -1729,7 +1729,7 @@ abstract class AppLocalizations {
   /// No description provided for @studysetFilesInTopicsHint.
   ///
   /// In en, this message translates to:
-  /// **'Or add one here: \"Add material → Upload a PDF\".'**
+  /// **'Or add one here: \"Add material → Upload a file\".'**
   String get studysetFilesInTopicsHint;
 
   /// Opens the card browser on every flashcard of a studyset's topics.
@@ -6793,13 +6793,13 @@ abstract class AppLocalizations {
   /// No description provided for @pdfUploadMenu.
   ///
   /// In en, this message translates to:
-  /// **'Upload a PDF'**
+  /// **'Upload a file'**
   String get pdfUploadMenu;
 
   /// No description provided for @pdfFileType.
   ///
   /// In en, this message translates to:
-  /// **'PDFs'**
+  /// **'PDFs and Markdown'**
   String get pdfFileType;
 
   /// No description provided for @pdfAdded.
@@ -6853,7 +6853,7 @@ abstract class AppLocalizations {
   /// No description provided for @filesEmptyHint.
   ///
   /// In en, this message translates to:
-  /// **'Use \"Add material → Upload a PDF\" to add one.'**
+  /// **'Use \"Add material → Upload a file\" to add one.'**
   String get filesEmptyHint;
 
   /// No description provided for @filesSort.
@@ -12877,6 +12877,126 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Answer the question above to go on.'**
   String get runnerScrollLocked;
+
+  /// A file named .md or .markdown that couldn't be read as text.
+  ///
+  /// In en, this message translates to:
+  /// **'This file isn\'t Markdown text in UTF-8.'**
+  String get markdownNotText;
+
+  /// A Markdown file over the server's limit for files.
+  ///
+  /// In en, this message translates to:
+  /// **'This Markdown file is larger than {megabytes} MB.'**
+  String markdownTooLarge(int megabytes);
+
+  /// How much of a Markdown file is read, or where in it something is, in percent
+  ///
+  /// In en, this message translates to:
+  /// **'{percent}%'**
+  String markdownShare(int percent);
+
+  /// Where in a Markdown file the cards or the note below are: a heading in the reader's panels
+  ///
+  /// In en, this message translates to:
+  /// **'At {percent}%'**
+  String markdownAt(int percent);
+
+  /// Segment in a Markdown file's cards panel: the cards grouped by where in the file they are (as Page does for PDFs)
+  ///
+  /// In en, this message translates to:
+  /// **'Place'**
+  String get readerByPlace;
+
+  /// Button in a Markdown file's cards panel: an assistant proposes flashcards from the part of the file in view
+  ///
+  /// In en, this message translates to:
+  /// **'From this part'**
+  String get proposalsFromPart;
+
+  /// Below a card being written beside a Markdown file: where in the file it is tied to
+  ///
+  /// In en, this message translates to:
+  /// **'Referenced place: {percent}%'**
+  String readerReferencedShare(int percent);
+
+  /// Button: ties the card to the place in the Markdown file in view
+  ///
+  /// In en, this message translates to:
+  /// **'Use current ({percent}%)'**
+  String readerUseCurrentShare(int percent);
+
+  /// A studyset's Progress: how much of a Markdown file is read
+  ///
+  /// In en, this message translates to:
+  /// **'{percent}% read'**
+  String progressMarkdownShare(int percent);
+
+  /// A Markdown file's reading deadline not started yet
+  ///
+  /// In en, this message translates to:
+  /// **'Starts in {days, plural, =1{1 day} other{{days} days}} — {percent}%/day planned'**
+  String markdownDeadlineUpcoming(int days, int percent);
+
+  /// A Markdown file's reading deadline that passed
+  ///
+  /// In en, this message translates to:
+  /// **'Deadline passed — {percent}% left'**
+  String markdownDeadlinePassed(int percent);
+
+  /// A Markdown file's reading deadline due today
+  ///
+  /// In en, this message translates to:
+  /// **'Due today — {percent}% left'**
+  String markdownDeadlineToday(int percent);
+
+  /// A Markdown file's reading deadline on its way: how much to read a day
+  ///
+  /// In en, this message translates to:
+  /// **'{percent}%/day to reach {target}% in {days, plural, =1{1 day} other{{days} days}}'**
+  String markdownDeadlinePace(int percent, int target, int days);
+
+  /// Help in the deadline sheet of a Markdown file
+  ///
+  /// In en, this message translates to:
+  /// **'Read a share of the file by a day. Progress follows the furthest you scroll as you read.'**
+  String get markdownDeadlineHelp;
+
+  /// Field in the deadline sheet of a Markdown file: the share to reach, in percent
+  ///
+  /// In en, this message translates to:
+  /// **'Reach (%)'**
+  String get markdownDeadlineReach;
+
+  /// A heading of a Markdown file offered as a deadline's target, with the share it ends at
+  ///
+  /// In en, this message translates to:
+  /// **'{title}  ·  to {percent}%'**
+  String markdownDeadlineChapterEntry(String title, int percent);
+
+  /// Chip on a Markdown file's deadline: how far to read today
+  ///
+  /// In en, this message translates to:
+  /// **'to {percent}% today'**
+  String markdownToShareToday(int percent);
+
+  /// A Markdown file's deadline: how far it is read of the share to reach, and when
+  ///
+  /// In en, this message translates to:
+  /// **'{furthest}%/{target}% · {when}'**
+  String markdownDeadlinesProgress(int furthest, int target, String when);
+
+  /// Tooltip: shows a Markdown file as written, rather than formatted
+  ///
+  /// In en, this message translates to:
+  /// **'Show the source text'**
+  String get readerShowSource;
+
+  /// Tooltip: shows a Markdown file formatted again, after its source text
+  ///
+  /// In en, this message translates to:
+  /// **'Show it formatted'**
+  String get readerShowFormatted;
 }
 
 class _AppLocalizationsDelegate