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/core/reminders.py

@@ -147,6 +147,7 @@ def build_ics_reminders(
     num_reminders: int = 3,
     now: Optional[datetime] = None,
     threshold: Optional[datetime] = None,
+    reminder_suffix: str = "(reminder {idx}/{total})",
 ) -> Optional[str]:
     """Build the ``.ics`` content with one VEVENT per reminder date.
 
@@ -169,6 +170,10 @@ def build_ics_reminders(
         num_reminders: requested reminder count (ADVANCED mode only).
         now: "today" reference; defaults to ``datetime.now()``.
         threshold: check-alive date (ADVANCED mode only; required there).
+        reminder_suffix: text appended to each event summary, with the
+            ``{idx}`` and ``{total}`` fields. This module stays free of
+            Electrum imports, so the GUI passes it already translated; the
+            English default keeps the CLI output unchanged.
 
     Returns:
         The ``.ics`` content string, or ``None`` when no reminder falls in the
@@ -210,7 +215,9 @@ def build_ics_reminders(
         # The visible date of this event: "offset" days before the deadline.
         event_dt = format_time(locktime - timedelta(days=offset))
         # Suffix the summary so the N events are easy to tell apart.
-        event_summary = ical_escape(f"{summary_base} (reminder {idx}/{total})")
+        event_summary = ical_escape(
+            "{} {}".format(summary_base, reminder_suffix.format(idx=idx, total=total))
+        )
         lines.extend([
             "BEGIN:VEVENT",
             # Offset in the UID keeps each event unique (no merging).