Agent: resolve "today" in the browser's timezone, not hardcoded CR

The CR-hardcoded today broke two scenarios: the owner traveling, and
any future non-CR user. Now the provider sends the browser's IANA
timezone (X-Client-Timezone) with every CopilotKit request — the
runtime forwards x-* headers to the agent on its own — and the backend
binds it to a per-request ContextVar next to the DB session.
get_current_date and the future-cuota bounds use it; the tool also
reports the timezone so the model can echo it. Unknown or absent zones
(n8n posts, tests) fall back to Costa Rica; comma-joined duplicate
header values are tolerated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/tests/test_agent_tools.py

@@ -214,3 +214,26 @@ def test_daily_spending_day_scoped(bound_session):
     assert out[0]["date"] == today.date().isoformat()
     assert out[0]["total_crc"] == 150.0
     assert out[0]["count"] == 2
+
+
+def test_get_current_date_respects_client_timezone():
+    from datetime import datetime, timezone as tz
+
+    from app.timeutil import (
+        client_tz,
+        reset_client_timezone,
+        set_client_timezone,
+    )
+
+    token = set_client_timezone("Asia/Tokyo")
+    try:
+        out = tools.get_current_date()
+        assert out["timezone"] == "Asia/Tokyo"
+        assert out["date"] == datetime.now(client_tz()).date().isoformat()
+    finally:
+        reset_client_timezone(token)
+
+    # Unknown names are ignored — CR fallback stays active.
+    assert set_client_timezone("Not/AZone") is None
+    assert set_client_timezone(None) is None
+    assert tools.get_current_date()["timezone"] == "UTC-06:00"