feat: a round in the order cards come, shared daily limits, and a start screen

Studying:
- A round is one list in the order its cards come: those done, the one
  shown, those to come. A card due again within the day goes back in
  after 4 more cards when rated Bad, at the end otherwise, so the pills
  above the card follow it; the buttons say "in 4 cards", "at the end",
  "now" (round_order.dart).
- The studied studyset's daily limits hold for everything inside it
  together, as Anki's do for subdecks: twenty topics start 20 new cards
  a day in all, not 20 each.
- The study header's studyset name shows whole on hover or long press.
- The assistant's button sits at the lower left: in the Library across
  from Create studyset, in a studyset above its bar, and higher while
  studying, above the buttons that answer the card.

A studyset:
- Progress also counts the learning paths' steps done and lists the
  PDFs not read to their end, begun first, each opening the reader,
  across its topics (MaterialProgressCubit).
- Next up starts with studying for the exam while one is ahead: its
  reviews due and the new cards its pace leaves for the day.
- Learning paths tagged with a level count towards its automatic exam
  plan and offer it, as cards do.
- A step's first heading equal to its title is dropped on import and
  hidden in paths imported before.

Signing out, or a session the server ended, leads to a start screen
(GitHub, email, without an account, another instance) rather than a new
anonymous account; only the very first start makes one by itself.
Settings names the kind of account and its instance.

Also: the Watch screen reads the cards once for held words and their
progress; its ±10 s buttons tick as the navigation bar does; a map card
keeps its view as its caption changes and brings a picked region
towards the middle; an empty or failed library can be pulled to sync.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

deploy/backups.md

@@ -33,9 +33,9 @@ then sync with a server that has forgotten their last hour:
 - **Someone who used the instance before the backup** loses nothing: their devices still
   have what the server lost, and send it again.
 - **Someone who started after the backup** is unknown to the server. At the next start,
-  their app finds its session gone, starts a new anonymous account and deletes the old
-  one's data from the device, the only copy left. The same happens to a device that
-  signed in after the backup, though a GitHub account can be signed into again.
+  their app finds its session gone and asks how to start; starting anew deletes the
+  old account's data from the device, the only copy left. The same happens to a device
+  that signed in after the backup, though a GitHub account can be signed into again.
 - **An account deleted after the backup** comes back. Delete it again if you know which
   one it was.
 - **A release published after the backup** is gone from the instance; learners who took