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_en.arb

@@ -330,7 +330,7 @@
   "studysetNotesInTopics": "The notes are in its topics",
   "studysetNotesInTopicsHint": "Or write one here: \"Add material → Create a note\".",
   "studysetFilesInTopics": "The files are in its topics",
-  "studysetFilesInTopicsHint": "Or add one here: \"Add material → Upload a PDF\".",
+  "studysetFilesInTopicsHint": "Or add one here: \"Add material → Upload a file\".",
   "studysetBrowseAllFlashcards": "{count, plural, =1{Browse it} other{Browse all {count}}}",
   "@studysetBrowseAllFlashcards": {"description": "Opens the card browser on every flashcard of a studyset's topics.", "placeholders": {"count": {"type": "int"}}},
   "studysetCreateFlashcards": "Create flashcards",
@@ -1326,8 +1326,8 @@
   "studyButtonGone": "That studyset no longer exists.",
   "pdfPageField": "PDF page",
   "studysetTabFiles": "Files",
-  "pdfUploadMenu": "Upload a PDF",
-  "pdfFileType": "PDFs",
+  "pdfUploadMenu": "Upload a file",
+  "pdfFileType": "PDFs and Markdown",
   "pdfAdded": "Added \"{title}\".",
   "@pdfAdded": {"placeholders": {"title": {"type": "String"}}},
   "pdfNotPdf": "This file isn't a PDF.",
@@ -1339,7 +1339,7 @@
   "pdfDuplicateKeep": "Keep the one there",
   "pdfDuplicateAdd": "Add a copy",
   "filesEmpty": "No files yet",
-  "filesEmptyHint": "Use \"Add material → Upload a PDF\" to add one.",
+  "filesEmptyHint": "Use \"Add material → Upload a file\" to add one.",
   "filesSort": "Sort",
   "filesSortCustom": "Custom order",
   "filesSortName": "Name (A–Z)",
@@ -2690,5 +2690,45 @@
   "runnerOutline": "Outline",
   "@runnerOutline": {"description": "The list of a learning path's steps beside or over its page in the Scroll view."},
   "runnerScrollLocked": "Answer the question above to go on.",
-  "@runnerScrollLocked": {"description": "Below the question a learning path's page stops at, in the Scroll view."}
+  "@runnerScrollLocked": {"description": "Below the question a learning path's page stops at, in the Scroll view."},
+  "markdownNotText": "This file isn't Markdown text in UTF-8.",
+  "@markdownNotText": {"description": "A file named .md or .markdown that couldn't be read as text."},
+  "markdownTooLarge": "This Markdown file is larger than {megabytes} MB.",
+  "@markdownTooLarge": {"description": "A Markdown file over the server's limit for files.", "placeholders": {"megabytes": {"type": "int"}}},
+  "markdownShare": "{percent}%",
+  "@markdownShare": {"description": "How much of a Markdown file is read, or where in it something is, in percent", "placeholders": {"percent": {"type": "int"}}},
+  "markdownAt": "At {percent}%",
+  "@markdownAt": {"description": "Where in a Markdown file the cards or the note below are: a heading in the reader's panels", "placeholders": {"percent": {"type": "int"}}},
+  "readerByPlace": "Place",
+  "@readerByPlace": {"description": "Segment in a Markdown file's cards panel: the cards grouped by where in the file they are (as Page does for PDFs)"},
+  "proposalsFromPart": "From this part",
+  "@proposalsFromPart": {"description": "Button in a Markdown file's cards panel: an assistant proposes flashcards from the part of the file in view"},
+  "readerReferencedShare": "Referenced place: {percent}%",
+  "@readerReferencedShare": {"description": "Below a card being written beside a Markdown file: where in the file it is tied to", "placeholders": {"percent": {"type": "int"}}},
+  "readerUseCurrentShare": "Use current ({percent}%)",
+  "@readerUseCurrentShare": {"description": "Button: ties the card to the place in the Markdown file in view", "placeholders": {"percent": {"type": "int"}}},
+  "progressMarkdownShare": "{percent}% read",
+  "@progressMarkdownShare": {"description": "A studyset's Progress: how much of a Markdown file is read", "placeholders": {"percent": {"type": "int"}}},
+  "markdownDeadlineUpcoming": "Starts in {days, plural, =1{1 day} other{{days} days}} — {percent}%/day planned",
+  "@markdownDeadlineUpcoming": {"description": "A Markdown file's reading deadline not started yet", "placeholders": {"days": {"type": "int"}, "percent": {"type": "int"}}},
+  "markdownDeadlinePassed": "Deadline passed — {percent}% left",
+  "@markdownDeadlinePassed": {"description": "A Markdown file's reading deadline that passed", "placeholders": {"percent": {"type": "int"}}},
+  "markdownDeadlineToday": "Due today — {percent}% left",
+  "@markdownDeadlineToday": {"description": "A Markdown file's reading deadline due today", "placeholders": {"percent": {"type": "int"}}},
+  "markdownDeadlinePace": "{percent}%/day to reach {target}% in {days, plural, =1{1 day} other{{days} days}}",
+  "@markdownDeadlinePace": {"description": "A Markdown file's reading deadline on its way: how much to read a day", "placeholders": {"percent": {"type": "int"}, "target": {"type": "int"}, "days": {"type": "int"}}},
+  "markdownDeadlineHelp": "Read a share of the file by a day. Progress follows the furthest you scroll as you read.",
+  "@markdownDeadlineHelp": {"description": "Help in the deadline sheet of a Markdown file"},
+  "markdownDeadlineReach": "Reach (%)",
+  "@markdownDeadlineReach": {"description": "Field in the deadline sheet of a Markdown file: the share to reach, in percent"},
+  "markdownDeadlineChapterEntry": "{title}  ·  to {percent}%",
+  "@markdownDeadlineChapterEntry": {"description": "A heading of a Markdown file offered as a deadline's target, with the share it ends at", "placeholders": {"title": {"type": "String"}, "percent": {"type": "int"}}},
+  "markdownToShareToday": "to {percent}% today",
+  "@markdownToShareToday": {"description": "Chip on a Markdown file's deadline: how far to read today", "placeholders": {"percent": {"type": "int"}}},
+  "markdownDeadlinesProgress": "{furthest}%/{target}% · {when}",
+  "@markdownDeadlinesProgress": {"description": "A Markdown file's deadline: how far it is read of the share to reach, and when", "placeholders": {"furthest": {"type": "int"}, "target": {"type": "int"}, "when": {"type": "String"}}},
+  "readerShowSource": "Show the source text",
+  "@readerShowSource": {"description": "Tooltip: shows a Markdown file as written, rather than formatted"},
+  "readerShowFormatted": "Show it formatted",
+  "@readerShowFormatted": {"description": "Tooltip: shows a Markdown file formatted again, after its source text"}
 }