fix(app): nothing secret is copied

The reading of the full reading's fixes found that copied recovery codes
and authenticator keys still reached clipboard histories (Windows, KDE's
Klipper, Gboard, Samsung's) and cloud clipboards, as no Flutter engine
marks a copy as sensitive, and that on Android an app in the background
can't empty the clipboard again, so the minute's clearing often never
happened.

As the developer decided: recovery codes are saved, like the phrase; an
authenticator's key is scanned as a QR code, opened in the authenticator
app, or typed from the screen. copySecret and its texts go.

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

@@ -82,9 +82,10 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   and the instance swaps anything in; deleting by a code by mail
   follows `secondFactorWorks`, as the instance tells it, and asks for the
   address only where the device doesn't know it. Secrets shown stay the
-  person's: the recovery phrase is saved or written down, never copied;
-  recovery codes and an authenticator's key are copied and the clipboard
-  emptied a minute later (`copySecret`); all of them show in a
+  person's: nothing secret is copied (no app can mark a copy as sensitive
+  through Flutter, so clipboard histories and cloud clipboards would keep
+  it): the recovery phrase and codes are saved or written down, an
+  authenticator's key scanned, opened or typed; all of them show in a
   `SecretScreen`, kept out of screenshots and recents on Android
   (`FLAG_SECURE`: the app's `MainActivity` answers `app.halkyon/screen_guard`,
   as Learn's and Todo's `ScreenGuard.kt` do) and covered on iOS while the app