feat(weather): default to Tân Thuận and accept lat,long coordinates

Forecast commands without a location now use phường Tân Thuận (formerly Quận 7), shared with the flood view, instead of the Ho Chi Minh City centre; city aliases still resolve to the city. A decimal "lat,long" argument skips geocoding and is used directly.

cmd/server/command_menu_test.go

@@ -96,10 +96,10 @@ func TestCommandDiscovery_AllPublicCommandsHaveSafeMetadata(t *testing.T) {
 		"stock_sell":           "<quantity> <ticker>",
 		"stock_cash_dividend":  "<vnd_per_share> <ticker>",
 		"stock_share_dividend": "<ratio(owned:new)> <ticker>",
-		"thoitiet":             "[location...]",
-		"thoitiethomnay":       "[location...]",
-		"thoitietngaymai":      "[location...]",
-		"thoitiettuannay":      "[location...]",
+		"thoitiet":             "[location... | lat,long]",
+		"thoitiethomnay":       "[location... | lat,long]",
+		"thoitietngaymai":      "[location... | lat,long]",
+		"thoitiettuannay":      "[location... | lat,long]",
 		"trongtruonghop":       "[target...]",
 		"tth":                  "[target...]",
 		"wheelofnames":         "<option,...>",