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_client/lib/migrations/migration_registry.dart

@@ -10,6 +10,7 @@ part '20260930145136285-reviews/migration.dart';
 part '20261001064926911-tree-tags-assets/migration.dart';
 part '20261002191122122-notes/migration.dart';
 part '20261002211346701-learning-paths/migration.dart';
+part '20261003110855698-pdf-sources/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -22,6 +23,7 @@ class MigrationRegistry {
     _Migration20261001064926911_tree_tags_assets(),
     _Migration20261002191122122_notes(),
     _Migration20261002211346701_learning_paths(),
+    _Migration20261003110855698_pdf_sources(),
   ];
 
   /// List of all client-side database migration versions.