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.

PLAN_I18N.md

@@ -225,6 +225,7 @@ Phases 1 and 2 change nothing visible, so they may be delivered as a single ZIP/
   - mark the English defaults with `N_()`;
   - when the stored value is missing **or equal to the English default**, use the translated default at the moment the text is used (calendar export, history label);
   - the tokens must survive translation.
+  - **Changed during Phase 2 (safety):** `HISTORY_LABEL` stays English and is *not* translatable. `Util._label_matches_history()` uses it to recognise BAL's own local transactions (stale-history cleanup, spendable UTXOs), so a label that changed with the GUI language would no longer match the transactions saved before. Only `EVENT_SUMMARY` and `EVENT_DESCRIPTION` follow the language (`BalConfig(..., translatable=True)`). The same rule applies to every stored or compared text (wallet labels, `heirs.TRANSACTION_LABEL`, the status history).
 - **2.10** `pyproject.toml`: add `"INT"` to `[tool.ruff.lint] select`.
   - Ruff's hint suggests `%` formatting: **do not follow it**. Use `.format()`, which is Electrum's rule.
   - `make-release.sh` treats Ruff errors as blocking, so INT must end up clean.