refactor(theme): the look comes from halkyon_ui

The theme, Noto Sans, the window sizes and the widgets every screen is
built of move to halkyon_ui (halkyon_shared), pinned to a signed commit
as the sync fork is, so Halkyon's other apps share them. Learn keeps its
own colours (the FSRS ratings, a starred star) as LearnColors beside the
theme, the CJK fonts with their licences, and its launch screen's word
and icon. Nothing on screen changes.

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

@@ -816,6 +816,15 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.1.3"
+  halkyon_ui:
+    dependency: transitive
+    description:
+      path: "packages/halkyon_ui"
+      ref: "80fa7b96286b7a00791c4b7ea42af408f4ea548a"
+      resolved-ref: "80fa7b96286b7a00791c4b7ea42af408f4ea548a"
+      url: "https://code.halkyon.app/project-halkyon/halkyon_shared.git"
+    source: git
+    version: "0.1.0"
   hex:
     dependency: transitive
     description: