feat: PDF sources: read, take notes and cards from pages, deadlines

PDFs live in a studyset's new Files tab: uploaded from Add material
(a file the studyset holds already asks first), each row with its
pages, cards, notes and size, how far it is read and its reading
deadline; renamed, given a deadline (a page or a chapter by a day),
marked as read, started afresh, deleted with a moment to undo.

The reader (/pdfs/:id) opens where reading left off: on wide windows
a rail of tools (search, fit, zoom, fullscreen), a page counter with
the chapter, and a sidebar; on phones a top bar, a page scrubber and
a sheet. The sidebar holds the PDF's flashcards, by page or chapter,
made from selected text or written there, each tied to its page and
tagged with its chapter; and the page's note, an ordinary note tied
to the page. Screenshots of a region become images that know their
page. Cards and screenshots lead back to their page; reading on past
a chapter with cards suggests studying it. Home shows the deadlines
running and to come, and a timeline of them.

Server: pdf_source rows (content in the payload, last writer wins)
and an append-only pdf_reading log, the furthest page derived from
it so devices never conflict. Files up to 50 MB for PDFs (20 MB for
others; a larger file that isn't a PDF is deleted once stored). A
deleted PDF takes its file's asset row along unless another PDF has
the same file. Card payload 4 and note payload 2 keep the page.

The reader draws with pdfrx (PDFium) behind PdfEngine in
Dependencies; widget tests use a fake, and
integration_test/pdf_engine_test.dart checks the real engine on a
device or the desktop.

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_server/test/integration/asset/asset_test.dart

@@ -156,6 +156,33 @@ void main() {
         );
       });
 
+      test('keeps a file over 20 MB only when it is a PDF, which may take '
+          'up to 50 MB', () async {
+        const size = AssetEndpoint.maxOtherBytes + 1;
+        final pdf = Uint8List(size)
+          ..setAll(0, utf8.encode('%PDF-1.7'))
+          ..fillRange(8, size, 32);
+        final other = Uint8List(size)..fillRange(0, size, 32);
+        final otherHash = sha256.convert(other).toString();
+
+        expect(await add(alice, pdf), isTrue);
+        expect(await add(alice, other), isFalse);
+
+        // What isn't a PDF is gone: the server asks for it anew.
+        expect(
+          await endpoints.asset.startUpload(signedIn(alice), otherHash, size),
+          isNotNull,
+        );
+        await expectLater(
+          endpoints.asset.startUpload(
+            signedIn(alice),
+            otherHash,
+            AssetEndpoint.maxBytes + 1,
+          ),
+          throwsArgumentError,
+        );
+      });
+
       test('finishes no upload that never came', () async {
         expect(
           await endpoints.asset.finishUpload(signedIn(alice), imageHash),