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/test/feature/studyset/widget/studyset_screen_test.dart

@@ -609,7 +609,7 @@ void main() {
         'Create a path',
         'Create flashcards',
         'Create a note',
-        'Upload a PDF',
+        'Upload a file',
       ])
         tester.getRect(find.text(item)).top,
     ];
@@ -816,7 +816,7 @@ void main() {
         'Files',
         LucideIcons.folder,
         'No files yet',
-        '"Add material → Upload a PDF"',
+        '"Add material → Upload a file"',
       ),
     ]) {
       await openPart(tester, part);