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>

pubspec.lock

@@ -1141,6 +1141,38 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.13.1"
+  pdfium_dart:
+    dependency: transitive
+    description:
+      name: pdfium_dart
+      sha256: e296a26c030e6e0fb34b32ce7481a3cfa0c2d35ce80f6269551c685759f4c91f
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.3.1"
+  pdfium_flutter:
+    dependency: transitive
+    description:
+      name: pdfium_flutter
+      sha256: "59897274513984b347fc98e7bf71ba457678daea8b5568cd7c688979aeb77765"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.3.1"
+  pdfrx:
+    dependency: transitive
+    description:
+      name: pdfrx
+      sha256: "52e78d84a706e5d63d2502689fe8e800d823bc780ac4341b2d3683a4d726505a"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.6.5"
+  pdfrx_engine:
+    dependency: transitive
+    description:
+      name: pdfrx_engine
+      sha256: f74ca9ea4ec722545f054a1d6ee1bf01222409ecd9458e5945c0cc5feaa3d2c6
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.6.1"
   petitparser:
     dependency: transitive
     description:
@@ -1294,6 +1326,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.2.1"
+  rxdart:
+    dependency: transitive
+    description:
+      name: rxdart
+      sha256: "5c3004a4a8dbb94bd4bf5412a4def4acdaa12e12f269737a5751369e12d1a962"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.28.0"
   scroll_to_index:
     dependency: transitive
     description: