feat(net): invio/ping/download verso Will-Executor in parallelo (anti-freeze)

Problema: i server Will-Executor venivano contattati in sequenza e, su
timeout, send_request riprovava 10x con sleep 3s (~130s per server morto).
Un solo server irraggiungibile bloccava l'intera operazione (impallamento UI).

Soluzione:
- ThreadPoolExecutor: ping e push ora in parallelo (tempo ~= server piu' lento,
  non la somma). Un server morto non blocca piu' gli altri.
- Fast-fail per operazioni interattive (ping/info/download): max_retries=0,
  niente retry-storm.
- Feedback live: callback on_each() aggiorna il dialog server-per-server
  (thread-safe via pyqtSignal di BalWaitingDialog.update).
- Push transazioni: parallelo ma con retry per-server mantenuti (no perdita tx).

File:
- bal/core/willexecutors.py: send_request(+max_retries,+retry_sleep),
  get_info_task fast-fail, NEW ping_servers_parallel(), push_transactions_parallel(),
  DEFAULT_TIMEOUT=5.
- bal/gui/qt/window.py: ping_willexecutors_task + push_transactions_to_willexecutors
  riscritti su helper paralleli con feedback live; fetch_will_executors_list fast-fail.
- bal/core/util.py: BUGFIX get_value_amount usava in_output (bool) invece di
  din_output (tupla) -> TypeError. Scoperto dai test ufficiali Gitea.

Test (contro il codice refactor):
- pytest tests/ ufficiali: 117 core + 65 gui = 182 passed.
- smoke/external_zip/windows_overflow/gui_fixes: OK.
- parallel_ping_test (nuovo): 0.50s per 8 server vs ~4.00s sequenziale.
- ruff: nessun nuovo problema introdotto (codice nuovo PEP8-compliant).

Aggiunti i test ufficiali del repo Gitea + REPORT_NETWORKING_PARALLELO.md.

tests/test_gui_calendar.py

@@ -0,0 +1,142 @@
+"""
+Tests for ``bal.gui.qt.calendar``.
+
+Covers BalCalendar static methods: format_time, ical_escape, fold_ical_line,
+write_temp_ics, open_with_default_app.
+
+Run:
+    QT_QPA_PLATFORM=offscreen python3 tests/test_gui_calendar.py
+"""
+
+import os
+import sys
+import tempfile
+from datetime import datetime, timezone
+
+sys.path.insert(0, __file__.rsplit("/", 2)[0])
+
+from bal.gui.qt.calendar import BalCalendar
+
+
+# ------------------------------------------------------------------ #
+# format_time
+# ------------------------------------------------------------------ #
+
+def test_format_time_utc():
+    dt = datetime(2025, 6, 1, 12, 30, 45, tzinfo=timezone.utc)
+    assert BalCalendar.format_time(dt) == "20250601T123045Z"
+
+
+def test_format_time_non_utc():
+    from datetime import timedelta
+    tz = timezone(timedelta(hours=2))
+    dt = datetime(2025, 1, 15, 8, 0, 0, tzinfo=tz)
+    result = BalCalendar.format_time(dt)
+    assert result.endswith("Z")
+    assert result == "20250115T060000Z"
+
+
+# ------------------------------------------------------------------ #
+# ical_escape
+# ------------------------------------------------------------------ #
+
+def test_ical_escape_no_change():
+    text = "hello world"
+    assert BalCalendar.ical_escape(text) == "hello world"
+
+
+def test_ical_escape_backslash():
+    assert BalCalendar.ical_escape("a\\b") == "a\\\\b"
+
+
+def test_ical_escape_semicolon():
+    assert BalCalendar.ical_escape("a;b") == "a\\;b"
+
+
+def test_ical_escape_comma():
+    assert BalCalendar.ical_escape("a,b") == "a\\,b"
+
+
+def test_ical_escape_multiline():
+    text = "line1\r\nline2"
+    result = BalCalendar.ical_escape(text)
+    assert "\r\n" in result
+    assert "line1" in result
+    assert "line2" in result
+
+
+def test_ical_escape_all():
+    text = "\\;,"
+    assert BalCalendar.ical_escape(text) == "\\\\\\;\\,"
+
+
+# ------------------------------------------------------------------ #
+# fold_ical_line
+# ------------------------------------------------------------------ #
+
+def test_fold_ical_line_short():
+    line = "SUMMARY:Test"
+    assert BalCalendar.fold_ical_line(line) == "SUMMARY:Test"
+
+
+def test_fold_ical_line_long():
+    line = "X-LONG:" + "a" * 100
+    result = BalCalendar.fold_ical_line(line, limit=75)
+    parts = result.split("\r\n ")
+    assert len(parts) > 1
+    assert result.startswith("X-LONG:")
+
+
+def test_fold_ical_line_unicode():
+    line = "DESCRIPTION:" + "\u20ac" * 40
+    result = BalCalendar.fold_ical_line(line, limit=75)
+    assert "\r\n " in result
+    assert "\u20ac" in result
+
+
+# ------------------------------------------------------------------ #
+# write_temp_ics
+# ------------------------------------------------------------------ #
+
+def test_write_temp_ics():
+    content = "BEGIN:VCALENDAR\r\nEND:VCALENDAR\r\n"
+    path = BalCalendar.write_temp_ics(content)
+    try:
+        assert os.path.isfile(path)
+        with open(path, "rb") as f:
+            assert f.read() == content.encode("utf-8")
+    finally:
+        os.unlink(path)
+
+
+def test_write_temp_ics_empty():
+    path = BalCalendar.write_temp_ics("")
+    try:
+        assert os.path.isfile(path)
+        with open(path, "rb") as f:
+            assert f.read() == b""
+    finally:
+        os.unlink(path)
+
+
+# ------------------------------------------------------------------ #
+# open_with_default_app
+# ------------------------------------------------------------------ #
+
+def test_open_with_default_app_not_found():
+    result = BalCalendar.open_with_default_app(
+        "/nonexistent/calendar_app", "/tmp/fake.ics"
+    )
+    assert result is False
+
+
+# ------------------------------------------------------------------ #
+# Main
+# ------------------------------------------------------------------ #
+
+if __name__ == "__main__":
+    for name in sorted(dir()):
+        if name.startswith("test_"):
+            globals()[name]()
+            print(f"  [OK] {name}")
+    print("[OK] All calendar tests passed")