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_flutter/test/support/fake_asset_gateway.dart

@@ -11,6 +11,9 @@ class FakeAssetGateway() implements AssetGateway {
   /// The rows, in the order they were stored.
   List<AssetRow> get rows => [..._rows.values];
 
+  /// The rows deleted, in order.
+  final deleted = <String>[];
+
   /// Stores [row] as if another device had written it and a sync had
   /// brought it here.
   void receiveRemote(AssetRow row) => _rows[row.id] = row;
@@ -39,4 +42,11 @@ class FakeAssetGateway() implements AssetGateway {
     );
     return id;
   }
+
+  @override
+  Future<void> delete(String id) async {
+    if (_rows.remove(id) != null) {
+      deleted.add(id);
+    }
+  }
 }