chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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_server/README.md

@@ -1,52 +0,0 @@
-# halkyon_learn_server
-
-This is the starting point for your Serverpod server.
-
-Start your server by running:
-
-    cd halkyon_learn
-    serverpod start
-
-When you are finished, you can shut down the running server with `Q`.
-
-## Dictionaries
-
-The dictionaries people look words up in (JMdict, …) are reference data the
-server holds for everyone. They come as dictionary files, which
-`halkyon_learn_curator` makes from the sources' downloads (its README says how).
-At every start, and every hour at seven past, the server loads the files in
-its `dictionaries` folder that changed since they were last loaded (by their
-SHA-256): in the background, a source at a time, and all at once, so searches
-meanwhile still find the old entries. A file it can't read is logged and left
-out. A dictionary whose file is gone is unloaded, once every file in the folder
-could be read (the curator removes the files of languages an instance no longer
-keeps). `DICTIONARIES_DIR` names another folder.
-
-EDRDG's licence asks a server showing its dictionaries to update them at least
-once a month:
-
-- **An instance** keeps them current by itself: the `dictionaries` service in
-  `deploy/compose.yaml` (the curator, in a container of its own) downloads
-  EDRDG's files of the day once its files are a week old, converts them, and
-  replaces them in a volume the server reads (`curate update`, the curator's
-  README); the server takes them up within the hour. To supply files yourself
-  instead, remove the service and mount `deploy/dictionaries/` into the server
-  (`compose.yaml` says how), and update them monthly.
-- **Development:** `dart run halkyon_learn_curator:curate update --into
-  ../halkyon_learn_server/dictionaries` in the curator's folder (about a
-  minute); the server takes them up within the hour.
-
-JMdict in English, with the JLPT levels and Tatoeba's example sentences, is
-about 20 MB gzipped and takes the server half a minute to load (219,000
-entries). KANJIDIC2 in English, with the JLPT levels of the kanji and the kana,
-is a dictionary of characters beside it (`kanjidic-en.jsonl.gz`, 0.7 MB,
-12,800 entries): the app's Kanji & kana tab. Their licences (CC BY-SA 4.0;
-CC BY 2.0 FR for the sentences; CC BY for the JLPT lists) ask for attribution:
-the app lists the credits each file carries.
-
-## Licence
-
-Copyright (C) 2026 The Halkyon Learn contributors
-
-Licensed under the Server Side Public License, version 1 (`SSPL-1.0`); the full text
-is in `LICENSE`.