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/sync/support/sync_devices.dart

@@ -16,6 +16,8 @@ import 'package:halkyon_learn_flutter/src/feature/learning_path/data/sync_path_a
 import 'package:halkyon_learn_flutter/src/feature/note/data/note_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/note/data/sync_note_gateway.dart';
 import 'package:halkyon_learn_flutter/src/feature/note/model/note.dart';
+import 'package:halkyon_learn_flutter/src/feature/pdf_source/data/pdf_source_repository.dart';
+import 'package:halkyon_learn_flutter/src/feature/pdf_source/data/sync_pdf_source_gateway.dart';
 import 'package:halkyon_learn_flutter/src/feature/settings/data/settings_repository.dart';
 import 'package:halkyon_learn_flutter/src/feature/settings/data/sync_settings_gateway.dart';
 import 'package:halkyon_learn_flutter/src/feature/study/data/review_repository.dart';
@@ -149,6 +151,9 @@ final class SyncDevice(
   /// The app's files on this device's database and cache; uploads only when
   /// told to.
   final AssetRepository assets,
+
+  /// The app's PDFs and their reading on this device's database.
+  final PdfSourceRepository pdfs,
 ) {
   /// Opens a device signed in as [person], whose local database in
   /// [database] claims [space] as its personal space.
@@ -176,6 +181,11 @@ final class SyncDevice(
     final paths = LearningPathRepository(
       SyncLearningPathGateway(db, syncRequests, person.authUserId.uuid),
     );
+    final assets = AssetRepository(
+      SyncAssetGateway(db, syncRequests),
+      ServerpodAssetServerGateway(client),
+      DirectoryAssetFileCache(Directory('${database.path}.files')),
+    );
     return SyncDevice(
       client,
       db,
@@ -193,11 +203,8 @@ final class SyncDevice(
         pathChanges: paths.changes,
       ),
       SettingsRepository(SyncSettingsGateway(db, syncRequests)),
-      AssetRepository(
-        SyncAssetGateway(db, syncRequests),
-        ServerpodAssetServerGateway(client),
-        DirectoryAssetFileCache(Directory('${database.path}.files')),
-      ),
+      assets,
+      PdfSourceRepository(SyncPdfSourceGateway(db, syncRequests), assets),
     );
   }
 
@@ -244,6 +251,7 @@ final class SyncDevice(
     await notes.dispose();
     await paths.dispose();
     await pathProgress.dispose();
+    await pdfs.dispose();
     await assets.dispose();
     await _syncRequests.dispose();
     await _db.close();