refactor(halkyon_app): the account section, About, the language and countries' names, shared

What Learn and Todo each held now comes from halkyon_app:
- AccountTiles: whose account on which instance, switching, signing
  out (asking first for an anonymous account, as it deletes it) and
  deleting it (asking first), the app's own words passed in.
- showAppAbout: the app's name, version, icon and legalese as it gives
  them, the source of what the instance runs (AppDependencies gains
  sourceCodeUrl), the app's own parts, the licence list.
- resolveAppLocale: the first preferred language the app speaks, never
  by country alone, British English otherwise.
- AssetCountryNames, the default of InstanceGate and InstanceScope:
  Unicode CLDR 48.2's territory names in English and German
  (tool/country_names.py, 7.6 KB), Hong Kong, Macao, Myanmar and
  Palestine by their short names as Learn's world map has them, so
  Todo says "Germany (EU)" where it said "DE (EU)".

Learn's texts read as before in every language; its About bloc and
gateway, its country names from the world map and the keys only the
moved code used are gone, as are Todo's copies.

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

packages/halkyon_app/README.md

@@ -25,16 +25,29 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   (ISO 3166-2, `US-CA`) as its operator says, "Germany (EU)" for a member of
   the European Union (`europeanUnionMembers`) and "California, United States"
   for a subdivision, with the most specific flag there is (a few subdivisions
-  have their own, such as Scotland; else the country's). Countries are named
-  by the app's `CountryNames`, subdivisions by Unicode CLDR's names in English
-  and German, which come with the package (`AssetSubdivisionNames`; the
-  package's `tool/subdivision_names.py` writes them).
+  have their own, such as Scotland; else the country's). Countries and
+  subdivisions are named by Unicode CLDR's names in English and German, which
+  come with the package (`AssetCountryNames`, `AssetSubdivisionNames`; the
+  package's `tool/country_names.py` and `tool/subdivision_names.py` write
+  them, from one release); tests give names of their own (`CountryNames`,
+  `SubdivisionNames`).
 - **Gates** (`lib/src/app/`): `InstanceGate` opens the app's dependencies on
   the instance in use (`AppDependencies`, handed down by `AppScope`), anew on
   a switch, the previous ones closed first; `SessionGate` keeps a person
   signed in, signing in anonymously at a first start and asking how to start
   once signed out (`StartScreen`); `PersonScope` opens each person's own
-  dependencies and closes them when the person goes.
+  dependencies and closes them when the person goes. `resolveAppLocale`
+  picks the app's language as every Halkyon app does: the first preferred
+  language it speaks, never by country alone, British English otherwise.
+- **About** (`showAppAbout`): the app's name (its frame's), its version,
+  icon and legalese where it gives them, where the source of what the
+  instance runs is (`InstanceSourceCode`, from `AppDependencies.sourceCodeUrl`;
+  both licences require offering it), the app's own parts (its credits), and
+  the licence list.
+- **The account** (`AccountTiles`): whose account on which instance,
+  switching to the account on another, signing out (which of an anonymous
+  account deletes it, so asks first) and deleting it (after asking), for an
+  app's settings, in the app's words where only it can say them.
 - **Signing in** (`lib/src/auth/`): `AuthRepository` on an `AuthGateway`;
   `ServerpodAuthGateway` signs in anonymously, with GitHub (the system
   browser and a loopback address on Linux and Windows, `LoopbackWebAuth`) and
@@ -70,12 +83,13 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
    delegates, beside `HalkyonUiLocalizations.delegate` and material_ui's
    `GlobalMaterialLocalizations.delegates`.
 4. **The frame** (`AppFrame`): the app's name and wordmark word, its icon, an
-   example address, its theme, delegates, languages and launch screen, and the
-   two sentences only it can say: what the first start's instance keeps of
-   the person's, and what a switch to another account leaves behind.
+   example address, its theme, delegates, languages (`resolveAppLocale` as
+   its resolution) and launch screen, and the three sentences only it can
+   say: what the first start's instance keeps of the person's, what a switch
+   to another account leaves behind, and where GitHub is added later.
 5. **The dependencies**: the app's composition root implements
-   `AppDependencies<P>` (`serverName`, `auth`, `signedOut`, `openPerson`,
-   `dispose`), its person's dependencies `Disposable`. Its `auth` is an
+   `AppDependencies<P>` (`serverName`, `auth`, `signedOut`, `sourceCodeUrl`,
+   `openPerson`, `dispose`), its person's dependencies `Disposable`. Its `auth` is an
    `AuthRepository` on `ServerpodAuthGateway(client, base:
    client.modules.base, callbackScheme: …, appName: …)`; the scheme is the
    app's own (`app.halkyon.learn`), registered with each platform and in the
@@ -88,7 +102,6 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
      frame: frame,
      choice: choice,
      checker: ServerpodInstanceChecker(app: 'todo'),
-     countryNames: …,
      readLook: …,           // the device's theme mode and language
      open: Dependencies.create,
      app: SessionGate(