i18n phase 3: Italian catalog

- babel.cfg and the catalog sources: bal/locale/bal.pot (424 texts) and
  bal/locale/it_IT/LC_MESSAGES/bal.po, fully translated (43 entries taken
  from Electrum's it_IT catalog, the rest following the owner's glossary
  and review: "locktime" kept in English, "transazione senza
  Will-Executor" for the backup transaction).
- build_zip.py compiles each bal.po into bal.mo inside the zip (Babel
  required); .po/.pot are not shipped and *.mo is ignored by git.
- tests/test_translations.py: catalogs compile, {} fields and $tokens
  match, no address/e-mail/URL added by a translation (patterns from
  electrum-locale).
- AGENTS.md: how to update the catalogs or add a language.

See CHANGELOG entry 60 and PLAN_I18N.md.

build_zip.py

@@ -9,7 +9,12 @@ Electrum loads external plugins from a ``.zip`` using Python's ``zipimport``.
   * uses standard DEFLATE compression (well supported by ``zipimport``);
   * emits entries in a deterministic, sorted order so the archive is
     reproducible (stable SHA-256);
-  * skips ``__pycache__`` directories and compiled ``*.pyc``/``*.pyo`` files.
+  * skips ``__pycache__`` directories and compiled ``*.pyc``/``*.pyo`` files;
+  * compiles each translation catalog ``bal/locale/<lang>/LC_MESSAGES/bal.po``
+    into ``bal.mo`` inside the archive (the ``.mo`` files are not kept in
+    git, and the ``.po``/``.pot`` sources are not shipped). This needs Babel
+    (``pip install babel``); without it the build stops, so a release can
+    never ship without its translations by mistake.
 
 The archive keeps the top-level ``bal/`` directory so that the package is
 importable as ``bal`` (and Electrum derives ``dirname='bal'`` from the path of
@@ -23,14 +28,36 @@ Prints the resulting size and SHA-256 so the download can be integrity-checked.
 """
 
 import hashlib
+import io
 import os
 import sys
+import time
 import zipfile
 
 SRC_ROOT = "bal"
 DEFAULT_OUT = "bal-electrum-plugin.zip"
 
 
+def compile_catalog(po_path: str) -> bytes:
+    """Return the compiled ``.mo`` bytes of the gettext catalog ``po_path``.
+
+    Fuzzy (unreviewed) entries are left out, as ``msgfmt`` does.
+    """
+    try:
+        from babel.messages.mofile import write_mo
+        from babel.messages.pofile import read_po
+    except ImportError:
+        raise SystemExit(
+            "ERROR: Babel is needed to compile the translations "
+            "(pip install babel)"
+        ) from None
+    with open(po_path, "rb") as f:
+        catalog = read_po(f)
+    buf = io.BytesIO()
+    write_mo(buf, catalog, use_fuzzy=False)
+    return buf.getvalue()
+
+
 def build(out_path: str) -> None:
     if os.path.exists(out_path):
         os.remove(out_path)
@@ -40,17 +67,34 @@ def build(out_path: str) -> None:
         # prune cache dirs in place so os.walk does not descend into them
         dirnames[:] = [d for d in dirnames if d != "__pycache__"]
         for fn in filenames:
-            if fn.endswith((".pyc", ".pyo")):
+            # .mo files are rebuilt from the .po sources below; the .po/.pot
+            # sources themselves are not shipped.
+            if fn.endswith((".pyc", ".pyo", ".po", ".pot", ".mo")):
                 continue
             files.append(os.path.join(dirpath, fn))
+    catalogs = {}  # archive path of the .mo -> source .po
+    for dirpath, _dirnames, filenames in os.walk(os.path.join(SRC_ROOT, "locale")):
+        for fn in filenames:
+            if fn.endswith(".po"):
+                po = os.path.join(dirpath, fn)
+                catalogs[po[: -len(".po")] + ".mo"] = po
     files.sort()
 
     with zipfile.ZipFile(
         out_path, "w", compression=zipfile.ZIP_DEFLATED, compresslevel=6
     ) as z:
-        for f in files:
+        entries = [(f, None) for f in files] + [
+            (mo, po) for mo, po in catalogs.items()
+        ]
+        for f, po in sorted(entries):
             arc = f.replace(os.sep, "/")  # forward slashes inside the archive
-            z.write(f, arc)
+            if po is None:
+                z.write(f, arc)
+            else:
+                # Same timestamp rule as z.write(): the source file's mtime.
+                info = zipfile.ZipInfo(arc, time.localtime(os.path.getmtime(po))[:6])
+                info.compress_type = zipfile.ZIP_DEFLATED
+                z.writestr(info, compile_catalog(po))
 
     # Integrity + summary
     with zipfile.ZipFile(out_path) as z:
@@ -63,7 +107,8 @@ def build(out_path: str) -> None:
 
     data = open(out_path, "rb").read()
     print(f"built : {out_path}")
-    print(f"files : {len(files)}")
+    print(f"files : {len(files) + len(catalogs)}")
+    print(f"langs : {', '.join(sorted(os.path.basename(os.path.dirname(os.path.dirname(m))) for m in catalogs)) or '-'}")
     print(f"size  : {len(data)} bytes")
     print(f"sha256: {hashlib.sha256(data).hexdigest()}")