i18n phases 1+2: BAL translation layer and translatable texts

Phase 1: new bal/i18n.py, BAL's own gettext layer (domain "bal", catalogs
read with plugin.read_file()). _() asks Electrum's catalog first, then
BAL's, then returns the English source. The Qt plugin loads the catalog of
Electrum's GUI language at start-up; the CLI stays English.

Phase 2: every user-visible GUI text is now a whole, extractable sentence
(Ruff INT rules enabled). Class-level texts are marked with N_() and
translated when shown. Stored data stays language-neutral: the status
history is written in English and translated for display, the calendar
defaults follow the GUI language, and the history label and wallet labels
are never translated because BAL uses them to recognise its transactions.

No visible change apart from the double colon fixed in the will detail.
See CHANGELOG entries 58 and 59 and PLAN_I18N.md.

bal/gui/qt/theme.py

@@ -87,7 +87,7 @@ def server_status_text(will_item) -> str:
     the user always knows whether each inheritance transaction is actually
     stored on the will-executor servers, regardless of the row colour.
     """
-    from electrum.i18n import _
+    from ...i18n import _
 
     if will_item.get_status("CHECK_FAIL") and not will_item.get_status("CHECKED"):
         return _("Not on server")
@@ -105,7 +105,7 @@ def server_status_text(will_item) -> str:
 def server_status_tooltip(will_item) -> str:
     """Return a detailed tooltip for the "Server" column, including the
     will-executor URL (if any) and the current server state."""
-    from electrum.i18n import _
+    from ...i18n import _
 
     url = None
     we = getattr(will_item, "we", None)